01 -
In a small bowl, stir brown sugar, flour, salt, and cinnamon together. Break up any sugar chunks with your fingers. Work in the cold butter until the mixture is crumb-like and keep it chilled until later.
02 -
Set the oven to 375°F and move the rack to the center. Coat a 9-inch square pan with nonstick spray or rub it with butter to grease.
03 -
Mix salt, baking powder, and flour in a medium bowl and put this aside.
04 -
Blend sugar and butter with a mixer until smooth and creamy, which will take about 2 minutes. Add eggs, one at a time, beating well as you go. Toss in lemon zest and vanilla extract, then mix them in too.
05 -
Slowly alternate adding the milk and flour blend to the wet mix, keeping the mixer on low. Carefully fold in the blueberries to spread them out evenly.
06 -
Pour the batter into the greased pan, spread it evenly, and sprinkle on the crumb topping. Bake for 40 to 45 minutes until golden and a tester comes out clean. Let the cake cool in the pan for around 20 minutes before serving.