Preheat the oven to 375 degrees F. Grease a 10-inch springform cake pan and set aside. Place the topping ingredients into a bowl and work together until it resembles cookie dough, then set aside. Place the 2 cups of flour, baking powder and salt for the cake into a medium bowl and whisk to combine. In another bowl, beat the butter and sugar until well-combined; beat in the eggs. Add the flour mixture in 3 parts, alternating with the buttermilk. Gently fold in the blueberries (be careful not to crush them). Spoon the batter into the prepared cake pan. Drop 1-teaspoonful amounts of the topping at various points on top of the cake batter. Bake for 55 to 60 minutes, or until a toothpick inserted into the centre of the cake pulls out clean. Cool the cake to room temperature, and then slice and serve. Garnish slices of the cake with whipped cream, mint sprigs and a few fresh blueberries, if desired.
Note: Coating the blueberries with flour helps to prevent them from sinking to the bottom of the cake as it bakes.