In a small bowl, whisk together the flour, baking powder, and salt and set aside.
In a mixing bowl, cream together the butter and sugar until light and fluffy, about 3 minutes.
Scrape down the bowl. Slowly stream in the whisked eggs and vanilla, mixing very well after each addition. Do not do this too quickly or the mixture will curdle. Add in half of the dry ingredients and mix on low until almost combined. Then mix in the buttermilk, followed by the remainder of the dry ingredients, mixing until just combined.
Spread the batter evenly into the prepared baking pan. Then evenly spread the blueberries over the batter. Finally, sprinkle the crumb topping over the berries.
Bake for 65-75 minutes, or until the middle is puffy and a toothpick inserted into the center comes out clean.
Let the cake cool for 20 minutes on a wire cooling rack, then remove it from the pan and let it cool completely.