To make the topping you will want to mix the sugar, flour and cinnamon in a small bowl. Cut the cold butter with a fork until it begins to crumble. Set aside (I like to place it in the refrigerator so it doesn’t melt into one big blob).
For the Batter:
In a large bowl whisk the flour, baking powder, and salt together. Set to the side.
Using a KitchenAid Stand Mixer fitted with a paddle attachment, cream the sugar and butter. Gradually add the egg and vanilla.
Next add the milk and the flour mixture alternating to the creamed mixture mixing only enough to blend. Remove the bowl from the stand and gently fold in the blueberries.
Pour the batter into a well-greased, 9-inch square cake pan. Sprinkle the topping over the batter and bake for 40 to 45 minutes or until cake comes out clean.
Let the cake cool for 10 minutes. Loosen the sides with a knife or spatula, cut, and serve.
Blueberries - You can use fresh or frozen blueberries for this recipe. If they are frozen you will need to thaw the blueberries and drain them very well before tossing in the cake.
Sinking Berries - If you find that your blueberries always sink in the cake you can toss them with 1 tablespoon of all-purpose flour before you fold them into the batter. This will help them from sinking.
Butter - Whenever making a crumb topping you want the butter to be cold. I find it's best to dice the butter which will help it break down into pea-size crumbs for the topping.
Crumb Topping - I like to make the crumb topping first then store in the refrigerator until ready to top the cake batter. When you make the topping you'll still have some loose flour/sugar mixture. That is totally okay and normal. It all won't be in the pea-size shape--about 75% will.