Preheat oven to 350 degrees.

Spray a 9 inch square baking pan with nonstick cooking spray.

For the filling: In a small bowl, combine hazelnuts, brown sugar, and cinnamon.

For the cake: In a large bowl, whisk together amaranth flour, potato starch, baking powder, baking soda, and salt.

In a medium bow, whisk together sugar, eggs, butter and vanilla until well blended. Whisk in buttermilk until blended.

Add the egg mixture to the flour mixture and stir until just blended. Gently fold in blueberries.

Spread half the batter evenly in prepared pan and sprinkle with filling. Dollop the remaining batter on top of the filling (it need not cover the filling completely).

Bake in preheated oven for 28-33 minutes or until a toothpick inserted in the center comes out clean. Let cool completely in pan on a wire rack.