I have had this recipe for many years and I don't recall where I got it from. I think it was fron some amish cookbook. This is a really good snack cake. It's great for breakfast or brunch. This goes good with a good cup coffee or glass of cold milk.
Place the dry streusel ingredients in a food processor bowl. Drop in butter and pulse until small crumbs form. This can be done in a bowl with a pastry blender too. Set aside.
Preheat oven to 375 degrees. In a large mixing bowl, cream the butter. Add the sugar and beat until light and fluffy. Add the egg, and beat well. Add the dry ingredients to the butter mixture alternating with the milk & vanilla. Mix well. Stir in the blueberries by hand. Spread the batter evenly in an oiled 8-inch square cake pan. Spoon the streusel topping evenly over the batter, and bake for 40-45 minutes. Let cake cool for 15 minutes and then cut into squares and serve warm.