Preheated oven to 350F
Coat inside of 8"x8" pan with butter, then dust with flour.
Combine butter and sugar in a large bowl, beat until light and fluffy.
Beat in one egg, and vanilla extract.
In another bowl, whisk flour, baking powder, and salt together.
In three parts, mix dry ingredients and buttermilk into wet ingredients (add 1/3 of dry + 1/3 of the buttermilk, mix until combined, the repeat two more times).
Gently mix blueberries into dough.
Spread dough evenly in baking pan.
Begin making streusel topping:
Combine flour, brown sugar, and cinnamon in a bowl.
In chunks, mix in butter with a fork or beater until you get a clumpy mixture.
Spread streusel topping on coffee cake.
Bake cake for 45-55 minutes, or until a toothpick comes out clean when piercing center of cake.
Allow cake to rest 10 minutes before enjoying!