With the rack in the middle position, preheat the oven to 180 °C (350 °F). Butter and flour a 25-cm (10-inch) tube pan.
In a bowl, combine all the dry ingredients and add the butter. Stir until the mixture has a grainy texture.
Spread and press in the bottom of the pan.
In a bowl, combine the blueberries and 30 ml (2 tablespoons) flour. Set aside in the freezer.
In another bowl, combine the remaining flour and baking powder. Set aside.
In a third bowl, cream the butter with the sugar and vanilla with an electric mixer. Add the eggs, one at a time, beating until smooth.
At low speed, add the dry ingredients alternately with the buttermilk. With a wooden spoon, add the blueberries and blend until they are well distributed in the batter.
Pour the batter into the prepared pan. Bake for about 1 hour 20 minutes or until a toothpick inserted in the centre of the cake comes out clean. Cool on a rack and unmould by inverting onto a large plate so that the oatmeal crisp is on top. Serve with blueberry sauce.
If you don’t have salted butter, replace it with unsalted butter and add a pinch of salt in the oatmeal crisp and 2.5 ml (1/2 teaspoon) salt in the mixture of dry ingredients of the cake.