Preheat oven to 350 degrees. Butter a 1-quart oval or square baking dish.
In the bowl of a stand mixer fitted with the paddle attachment or a large mixing bowl using an electric handheld mixer, cream together butter and 1/2 cup sugar.
Add egg and egg yolk, combining fully. Add salt and baking powder, combining fully. Finally add flour, scraping down the sides of the bowl as you mix.
In a small bowl, toss halved blackberries with 2 tablespoons sugar.
Gently fold sugared blackberries into batter, discard any leftover sugar.
Spread into baking dish evenly.
Bake for 35-45 minutes or until your buckle passes the toothpick test. Allow to rest for 10-20 minutes. Serve with ice cream.
If you've tried this recipe, please come back and let us know how it was!