Preheat oven to 375˚. Spray a 10-inch tart pan with a removable bottom with baking spray. You can also use a 9-inch cake pan, greased and lined with parchment paper.

With a whisk (I like this one) or wooden spoon, mix butter and sugar until light and fluffy, about one minute.

Add egg and mix well. Add buttermilk and mix until well incorporated.

Add one cup flour. Sprinkle baking powder and salt evenly over the flour. Mix just until all flour is incorporated. Add second cup of flour and again, mix just until all flour is incorporated.

Gently fold in blueberries.

For the topping, combine all ingredients and mix with a fork until large clumps form. Sprinkle topping evenly over cake batter with your hand, trying to keep dough in fairly large clumps.

Bake in middle of oven for 20 minutes, then cover with foil and return to oven. Bake for another 15-20 minutes until toothpick inserted in the center, comes out clean.

When cool sprinkle with powdered sugar, if desired.