Preheat the oven to 350 °F (175 °C). Line an 8-inch square metal pan with parchment, leaving an overhang on either side.
In a medium bowl, combine the milk and 2 cups buckwheat flakes and let sit for about 5 minutes.
Meanwhile, in the bowl of an electric mixer fitted with the paddle attachment, cream the butter and the sugars.
Add the egg, beat again, scraping down the sides as needed, and then add the vanilla. Beat until creamy and smooth.
Add the buckwheat flake/milk mixture and beat.
In a separate bowl, whisk together the flours, starches, baking soda and powder, salt, and the spice, and then add them into the mixer bowl. Beat on low to combine, then increase the speed to medium for one more minute.
Pour the batter into the prepared pan, sprinkle with 2 tablespoon cane sugar and 2 tablespoon buckwheat flakes, then scatter with raspberries, and top with the remaining cane sugar and flakes.
Bake for about 55 minutes or until a cake tester inserted into the center comes out clean.
Let cool before serving.
Prepare a raspberry topping before serving by lightly mashing the extra cup of berries with a tablespoon of sugar. Top pieces of the cake with this chunky raspberry sauce.