Directions

1. 1Make the cake: Preheat oven to 375°F. Lightly coat a 9-inch square baking pan with cooking spray; line bottom and sides of pan with parchment paper, leaving a 1-inch overhang on two opposite sides. Lightly coat parchment with cooking spray. Set aside.
2. 2Whisk together flour, baking powder, salt, and nutmeg in a medium bowl; set aside. Beat sugar and butter in a large bowl with an electric mixer on medium speed until fluffy, 2 to 3 minutes. Add egg, egg yolk, and vanilla; beat until smooth, about 30 seconds, stopping to scrape down sides of bowl as needed. With mixer on medium-low speed, add flour mixture alternately with milk in 3 additions, beginning and ending with flour mixture, beating until just combined after each addition. Gently fold in 2 1/2 cups blueberries. Spread batter in prepared pan. Scatter remaining 1/2 cup blueberries over top.
3. 3Make the streusel: Stir together flour, sugar, cinnamon, nutmeg, and salt in a medium bowl. Add butter, and incorporate using a pastry blender or your fingertips until mixture is crumbly. Sprinkle streusel evenly over batter in baking pan.
4. 4Bake until a wooden pick inserted into center of cake comes out clean, 40 to 45 minutes, rotating pan halfway through baking time. Let cool in baking pan on a wire rack for about 20 minutes. Using parchment paper overhang as handles, remove cake from pan; discard parchment paper. Serve warm, or let cool completely on wire rack.