For the cake:
8 tablespoons (1 stick) unsalted butter, at room temperature, plus more for pan
1 cup all-purprose flour, plus more for pan
½ cup cornmeal
2 teaspoons baking powder
½ teaspoon salt
1 cup sugar
2 large eggs
¼ teaspoon vanilla extract
¼ teaspoon freshly grated lemon zest
⅓ cup sour cream
2 cups blueberries, rinsed and patted dry
For the streusel:
½ cup sugar
6 tablespoons all-purpose flour
2 tablespoons cornmeal
¼ teaspoon ground cinnamon
Pinch of table salt
2 tablespoons unsalted butter, cut into small pieces
Heat oven to 350º. Line an 8-inch square cake pan with a piece of parchment, leaving an overhang on two sides of the pan. Butter and flour the bottom and sides of the pan, or lightly coat them with nonstick spray.
In a medium bowl, whisk together the flour, cornmeal, baking powder and salt. Beat the butter and sugar in the bowl of an electric mixer for about 2 minutes, or until light and fluffy. Add the eggs one at a time, beating well and scraping down the bowl after each addition. Add the vanilla and lemon zest and beat until combined. Add a third of the flour mixture, all of the sour cream and then another third of the flour, beating until just combined. Toss the blueberries with the remaining flour (this will prevent the blueberries from sinking). Using a rubber spatula, gently fold the blueberry-flour mixture into the cake batter.
Spread the cake batter into the prepared pan. Whisk together the sugar, flour, cornmeal and cinnamon in the medium bowl from earlier, then use a fork or your fingertips to work in the butter pieces. Scatter the streusel over the cake batter.
Bake the cake until the top is golden and a tester inserted in the center comes out clean, 35-55 minutes. Allow the cake to cool in the pan for 5-10 minutes, then use the parchment handles to lift it out of the pan and onto a wire cooling rack. Cool completely before cutting (if you can wait that long!).