- <b>For the crumb topping:</b>
- 3 1/2 cups cake flour (not self-rising)
- 2/3 cup granulated sugar
- 2/3 cup packed dark-brown sugar
- 1 1/2 teaspoons ground cinnamon
- Coarse salt
- 2 1/2 sticks unsalted butter, melted
- <b>For the cake:</b>
- 1 1/2 sticks unsalted butter, room temperature, plus more for pan
- 2 1/2 cups cake flour (not self-rising)
- 1/2 teaspoon baking soda
- Coarse salt
- 1 cup granulated sugar
- 2 large eggs plus 2 large egg yolks
- 1 teaspoon pure vanilla extract
- 2/3 cup low-fat buttermilk
- 1 cup store-bought blueberry jam (optional)
- Confectioners' sugar, for dusting
- Make the crumb topping: Mix together flour, sugars, cinnamon, and 1/2 teaspoon salt in a medium bowl. Pour warm melted butter over mixture, and mix using your hands until medium to large clumps form.
- Preheat oven to 325 degrees. Make the cake: Butter a 9-by-13-inch baking pan. Whisk together flour, baking soda, and 1/2 teaspoon salt in a medium bowl.
- Beat butter and granulated sugar with a mixer on medium speed until pale and fluffy, about 2 minutes. Beat in eggs and yolks, 1 at a time, then vanilla. Beat in flour mixture in 3 additions, alternating with buttermilk, beginning and ending with flour. Continue to beat until well combined.
- Spoon batter into pan, and spread evenly using an offset spatula. Spread with jam, if using. Sprinkle crumb-topping mixture evenly over top.
- Bake until golden brown and a toothpick inserted into the center comes out clean, about 1 hour. Transfer pan to a wire rack. Let cake cool slightly, about 15 minutes. Dust with confectioners' sugar. Serve warm or at room temperature.
Coffee cake can be stored at room temperature for up to 5 days.