For the cake:
2 cups cake flour
2 tsp. baking powder
¼ tsp. salt
1 vanilla bean, split lengthwise
11 tbsp. unsalted butter, at room temperature
1 1/3 cups sugar
3 large eggs plus 1 large egg yolk, at room temperature
¾ cup plus 2 tbsp. buttermilk, at room temperature
2 tsp. vanilla extract
For the blueberry sauce:
1 cup blueberries
3 tbsp. sugar
1¼ tsp. cornstarch
2 tsp. cold water
2 tsp. freshly squeezed lemon juice
For the strawberry sauce:
4 oz. strawberries, hulled, rinsed and coarsely chopped
1½ tbsp. sugar
1 tsp. freshly squeezed lemon juice
2½ cups vanilla bean ice cream, slightly softened (homemade or store-bought)
For the whipped cream frosting:
2 cups heavy cream
½ cup confectioners’ sugar
1 tsp. vanilla extract
Fresh strawberries and blueberries, for garnish
To make the cake, preheat the oven to 350˚ F. Line the bottom of two 8-inch round cake pans with parchment paper. Grease and flour the sides of the pan, shaking out the excess.
In a medium bowl, combine the cake flour, baking powder and salt. Whisk together and set aside. Add the butter to the bowl of an electric mixer fitted with the paddle attachment. Scrape the vanilla bean seeds into the bowl of the mixer with the butter and discard the pod (or reserve for another use.) Beat on medium-high speed for 3 minutes, until light and creamy in color. Scrape down the sides of the bowl and beat for one more minute.
Add the sugar to the butter mixture, about ¼ cup at a time, beating 1 minute after each addition. Mix in the eggs and egg yolk one at a time until incorporated. Scrape down the sides of the bowl after each addition. Combine the buttermilk and the vanilla extract in a liquid measuring cup. With the mixer on low speed, add the dry ingredients alternately with the wet ingredients, beginning and ending with the dry ingredients and mixing just until incorporated. Scrape down the sides of the bowl and mix for 15 seconds longer.
Divide the batter between the prepared pans and bake, rotating the pans halfway through, until the tops are golden and a toothpick inserted in the center comes out clean, about 20-22 minutes. Let cool in the pans about 10 minutes, then gently run a knife around the edge of the cake and turn the layers out onto a wire rack. Remove the parchment paper and allow to cool completely. Cover tightly with plastic wrap and chill the cake layers until you are ready to assemble the cake.
To make the blueberry sauce, combine the blueberries and sugar in a nonreactive saucepan over medium-high heat. Cook, stirring occasionally, until the berries begin to release their juices. In a small bowl combine the cornstarch with the water and lemon juice and whisk until smooth. Stir the cornstarch mixture into the blueberry mixture. Bring to a boil, then reduce the heat to a simmer and cook for 1 minute more. Remove from the heat. Cover and chill.
To make the strawberry sauce, combine the strawberries and sugar in a bowl. Toss well to combine. Mash the berries slightly with the tines of a fork. Stir in the lemon juice, cover and chill. Let the berries macerate at least one hour and if needed, mash once more to achieve your desired texture.
To assemble the cake, level the cake layers if necessary. Place one of the chilled cake rounds in the bottom of an 8-inch springform pan. Place a piece of parchment or wax paper around the circumference of the cake to act as a tight collar for the ice cream layer.* Be sure the ice cream is soft enough to spread smoothly and easily before proceeding. Spread the vanilla ice cream in an even layer over the bottom cake layer.
Spoon the blueberry and strawberry fillings on top of the ice cream in whatever pattern you prefer. (You may have a bit of each type of filling left over. I dare you to not eat it with a spoon!) Place the remaining cake layer on top of the ice cream and fruit filling. Ensure that the parchment collar is pulled tight around the cake and ice cream to keep the layers evenly in place. Close the sides of the springform around the assembled cake and transfer to the freezer to set. (Covering the cake at this point is not really necessary but if you plan to wait a long time before finishing the cake, I recommend covering it.)
When you are ready to frost the cake, place the heavy cream in the bowl of an electric mixer fitted with the whisk attachment. Whip on medium-high speed, gradually adding the confectioners’ sugar, until stiff peaks form. (Be careful not to overbeat!) Blend in the vanilla. Remove the sides of the springform pan carefully and peel away the parchment collar. Frost the top and side of the cake with an offset or straight spatula. Place the remaining whipped cream in a pastry bag fitted with a decorative tip and add additional accents as desired. (I used an unlabeled large fine star tip, similar to an Ateco #864 tip.) Garnish with fresh strawberries and blueberries. (The cake can be served at this point if desired.)
Store the cake covered in the freezer until ready to serve. Set the cake out and let stand at room temperature about 20 minutes before slicing. Run a long, thin knife until hot water to warm the blade. Wipe the blade completely dry and slice the cake, wiping the blade clean between slices and rewarming as needed.
*The parchment collar is not a necessity but I prefer it for cleaner edges of the ice cream layer. It also helps in the situation that your cake rounds are not exactly the same size as your springform pan, and if your entire cake is taller than your springform (it likely will be).