For the doughnuts:
2½ tsp. instant (rapid rise) yeast
2/3 cup milk, at room temperature
3½ cups all-purpose flour, plus more as needed
1/3 cup sugar
2 tsp. coarse salt
3 large eggs, at room temperature
7 tbsp. butter, at room temperature, cut into large chunks
For the filling:
1 quart strawberries, hulled
1/3 cup sugar
2 tbsp. fresh lemon juice
Canola oil, for frying
1 cup vanilla sugar*
In the bowl of a mixer fitted with the dough hook, combine the yeast with the milk and whisk briefly to dissolve. Add in the flour, sugar, salt, and eggs. Continue mixing on low speed 2-3 minutes more. Add in the butter a piece at a time, waiting until each is mostly incorporated before adding another. Continue kneading about 5-6 minutes more. If necessary, add extra flour 2 tablespoons at a time until the dough is cohesive but still slightly sticky.
Lay out a piece of plastic wrap and spray lightly with cooking spray. Transfer the dough to the plastic wrap, wrap tightly, and refrigerate overnight (at least 6 hours and up to 15 hours).
Lightly flour a baking sheet, and generously flour a work surface. Turn the dough out onto the work surface and gently pat or roll out to a slab about ½-¾ inch-thick. Use a 3 to 3½-inch biscuit or cookie cutter to cut out rounds of the dough. Transfer dough rounds to the prepared baking sheet, rerolling and recutting dough scraps as needed. Cover loosely with lightly oiled plastic wrap and let rise until puffy and nearly doubled, about 2-3 hours.
Meanwhile, make the strawberry jam filling. Coarsely chop the berries (use the food processor or blender, if you wish.) Transfer the chopped berries to a large skillet with the sugar and lemon juice. Cook over medium heat, stirring frequently, until the jam is thickened and bubbles cover the surface, about 15-20 minutes. Let cool well before using.
When you are ready to finish the doughnuts, fill a large saucepan or pot with canola oil to a depth of 3 inches. Clip a thermometer to the side and heat the oil to 350˚ F. Meanwhile, line a cooling rack with paper towels. When the oil has reached the appropriate temperature add doughnuts to the oil in the pan in a single layer so that they are not crowded. Fry, turning once, until golden on both sides, about 4-5 minutes total.
Use a skimmer or slotted spoon to transfer the finished doughnuts to the cooling rack. Place the vanilla sugar in a shallow plate or dish. After a few minutes of cooling, evenly coat all sides of the doughnuts in the vanilla sugar, shaking away the excess. Repeat the frying process until all doughnuts are cooked.
Transfer the strawberry filling to a pastry bag fitted with a plain round tip. Gently poke the pastry bag into each doughnut and squeeze a few tablespoons of filling into each. Serve fresh.
*If you don’t have vanilla sugar, no worries. Just use regular sugar. It will still taste fab. To make vanilla sugar, add spent vanilla pods to an airtight container of sugar and let stand at least 2 days. That’s it!