01 - Preheat oven to 350°F. Grease a 9-inch round or square baking pan and line it with parchment paper.
02 - Combine flour, sugar, and cubed butter in a small bowl. Blend with fingers or fork until coarse crumbs form. Set aside.
03 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
04 - In a large bowl, beat softened butter and sugar until light and fluffy, about 2 to 3 minutes.
05 - Add eggs one at a time, beating after each. Stir in lemon zest, lemon juice, and vanilla extract.
06 - Add half of the dry mixture to the wet ingredients, then milk, followed by remaining dry ingredients. Mix until just combined to avoid overmixing.
07 - Gently fold in blueberries, tossing frozen berries with 1 tablespoon flour beforehand if using frozen to prevent bleeding.
08 - Pour batter into prepared pan, smoothing the surface. Evenly sprinkle crumb topping over batter.
09 - Bake 40 to 45 minutes, or until a toothpick inserted in the center comes out clean. Cool in pan 15 minutes before transferring to a wire rack to cool completely.
10 - Whisk powdered sugar with lemon juice until smooth and pourable. Drizzle glaze over cooled cake before serving.