5 tablespoons (40 grams) all-purpose flour
1/2 cup (100 grams) granulated sugar
1 teaspoon ground cinnamon
4 tablespoons (1/4 cup, 2 ounces or 55 grams) unsalted butter, cold is fine
Pinch of salt
2 cups minus 1 tablespoon (i.e. 1 3/4 cups + 3 tablespoons or 240 grams) all-purpose flour
2 teaspoons baking powder
1/2 teaspoon table salt
4 tablespoons (1/4 cup, 2 ounces or 55 grams) unsalted butter, softened
3/4 cup (150 grams) granulated sugar
Zest of 1 lemon
1 large egg
1 teaspoon vanilla extract
1 pint (2 to 2 2/3 cups, 12 to 16 ounces, or 340 to 455 grams; see Note) fresh blueberries, clean and dry
1/2 cup milk, whole is ideal, any kind should work
1/2 cup (55 grams) walnuts, chopped medium fine (optional)
Confectioners’ sugar, for dusting (optional)
Heat oven to 375°F. Butter a 9-inch round baking pan (with at least 2″ sides) and dust it lightly with flour; line it with a round of parchment paper.
Prepare the topping by mixing the flour, sugar, cinnamon and salt, then cutting the butter in with a pastry blender, fork or your fingertips until the mixture resembles coarse crumbs. Set aside.
In a medium bowl, whisk flour, baking powder, and salt until combined. In a large bowl, beat butter, sugar and zest together until light and fluffy. Add egg and vanilla and beat until combined. Beat in 1/3 of the dry ingredient mixture until just combined, followed by 1/2 the milk; repeat with remaining dry ingredients and milk, finishing with the dry mixture. The batter will be very stiff, but don’t fret. Fold blueberries into cake batter until evenly distributed.
Scoop cake batter into prepared pan and smooth so that it is flat. If using walnuts, scatter them on top. Sprinkle with prepared streusel. Bake in heated oven for 40 minutes, or until a toothpick inserted into the middle of the cake comes out batter-free. You can let the cake cool complete in the pan on a rack, or just cool it in the pan for 20 minutes before flipping it out onto a cooling rack, removing the parchment paper lining, and flipping it back onto a plate. Dust with confectioners’ sugar, if using.
Do ahead: Cake keeps covered with plastic or foil at room temperature for three days. If longer, it might be best to keep it in the fridge. It gets more moist each day.