Preheat a toaster oven to 350 degrees F (175 degrees C), according to manufacturer's instructions. Spray a single-serving cake pan with cooking spray. Line with parchment paper.
Mix almond flour, powdered erythritol, baking powder, cinnamon, and stevia powder together in a bowl with a fork, ensuring no lumps remain.
Pour egg and melted butter into the flour mixture and combine well. Stir in 1/2 of the blueberries and raspberries.
Pour batter into the prepared cake pan; add remaining blueberries and raspberries and gently press them into the top of the batter. Hit the pan gently onto the counter to remove air bubbles and ensure all berries are covered with the mixture.
Bake in the preheated toaster oven until a toothpick inserted into the center comes out clean, about 20 minutes.
