Preheat oven to 325°F (160°C). Line an 8x8-inch (20x20 cm) baking pan with parchment paper leaving an overhang for easy removal; lightly grease the parchment with a little butter or nonstick spray.

Combine dry ingredients

In a medium bowl, sift or whisk together the unsweetened cocoa powder, baking cocoa (if using), powdered erythritol, granulated erythritol, kosher salt, and baking powder until uniform and free of lumps. Set aside.

Wet Ingredients

Mix wet ingredients

In a separate bowl, whisk the room-temperature egg until lightly beaten. Add the melted butter (make sure it is warm, not hot), heavy cream, peppermint extract, and vanilla extract. Whisk until well combined.

Combine wet and dry

Make a well in the center of the dry mixture and pour in the wet ingredients. Stir with a spatula or wooden spoon until just combined and smooth, scraping the sides and bottom. The batter should be thick and glossy; if it seems too dry, add 1 to 2 tablespoons additional cream.

Fold in chocolate chips (optional)

Fold in the mini sugar-free dark chocolate chips, if using, until evenly distributed.

Bake

Pour the batter into the prepared pan and smooth the top with an offset spatula. Bake in the preheated oven for 18–24 minutes, or until the center is set but still slightly fudgy. A toothpick inserted into the center should come out with a few moist crumbs (not wet batter).

Cool

Remove the pan from the oven and place on a wire rack. Let cool completely in the pan (about 60–90 minutes) so the sugar-free sweeteners set and the texture firms. For quicker setting, chill the pan in the refrigerator for 30–45 minutes after it has come to room temperature.

Finishing & Serving

Slice and garnish

Use the parchment overhang to lift the slab from the pan. Dust the top lightly with the reserved teaspoon of unsweetened cocoa powder using a fine sieve. Using a sharp knife warmed under hot water and dried, slice into 12 small squares for richer bites or 9 larger pieces. Wipe the knife between cuts for clean edges.

Garnish

Top each piece with a fresh mint leaf if desired. Serve at room temperature or slightly chilled. Store leftovers in an airtight container in the refrigerator up to 5 days; bring to room temperature before serving for best texture.