Preheat your oven to 350°F. Grease a 9-inch square baking pan or line it with parchment paper.

In a large bowl, whisk together the flour, sugar, and baking powder. Add the cold butter pieces and cut them into the flour mixture using a pastry blender or your fingers until the butter is in pea-sized bits.

Add the egg, milk, and lemon zest. Stir just until the dry ingredients are moistened. The batter will be very thick; use your hands to incorporate any remaining flour if needed.

Gently fold in the blueberries. Because the batter is dense, take your time to distribute them evenly. Spread the batter into the prepared pan.

In a small bowl, combine the sugar, flour, walnuts, and cinnamon. Add the cold butter cubes and work them into the mixture with your fingers or a fork until the topping is crumbly. Sprinkle evenly over the cake batter.

Bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan on a wire rack for at least 15 minutes.

For the glaze, whisk together the powdered sugar and milk until smooth. If the glaze is too thick, add a few drops of milk until it reaches a drizzling consistency. Drizzle over the warm cake and let set before slicing.