For the streusel:
¾ cup (5¼ oz.) granulated sugar
½ cup (1¾ oz.) whole wheat pastry flour
¼ cup (1 oz.) unbleached all-purpose flour
¼ cup (7/8 oz.) old-fashioned rolled oats
1 tsp. ground cinnamon
¼ tsp. grated nutmeg
¼ tsp. salt
6 tbsp. (3 oz.) unsalted butter, at room temperature
For the cake:
2 cups (8 oz.) white whole wheat flour
½ cup (2 1/8 oz.) unbleached all-purpose flour
2 tsp. baking powder
½ tsp. salt
4 tbsp. (2 oz.) unsalted butter, at room temperature
1 cup (7 oz.) granulated sugar
2 large eggs
½ cup (4 oz.) milk
1 tsp. vanilla extract
2 cups (10 oz.) blueberries*
Preheat the oven to 375˚ F. Line a 9 x 9-inch baking pan with parchment paper. To make the streusel, combine the sugar, whole wheat pastry flour, all-purpose flour, oats, cinnamon, nutmeg and salt in a medium bowl. Cut the butter into small pieces and toss in the bowl with the dry ingredients. Work together with your fingertips (or a pastry cutter) until the mixture is crumbly. Set aside.
To make the cake, combine the flours, baking powder, and salt in a medium bowl. In the bowl of an electric mixer, combine the butter and sugar. Beat until light and fluffy. Blend in the eggs one at a time, scraping down the sides of the bowl as needed. Stir in half of the dry ingredients just until incorporated, then the milk and vanilla, and then the remaining dry ingredients. Gently fold in the berries with a spatula.
Spread the batter into the prepared pan and sprinkle the streusel mixture over the top. Bake until a toothpick inserted in the center comes out clean and the top is golden brown, about 45-50 minutes. Transfer to a wire rack and let cool before slicing and serving.
*Fresh berries are preferred, but if you use frozen, do not thaw them first. Other fruits can be substituted as well, such as cherries, raspberries, sliced peaches, nectarines, etc.