Preheat the oven to 350°F (180°C). Line a 6-inch (15 cm) springform pan with parchment paper.

Stir 2 tablespoons water into the flaxseed in a small bowl. Cover and set aside for 10 minutes.

Melt the coconut oil. Transfer to a food processor. Add the beans, cocoa, sugar, baking powder, and salt and process until smooth.

Coarsely chop the chocolate and fold into the batter. Spread the batter evenly into the prepared pan.

Mix together the peanut and almond butters and drizzle over the batter. Use a fork or wooden skewer to swirl the nut butters in a marbleized pattern across the batter.

Bake for 40 minutes, or until the batter is just set to the touch. Cool completely on a wire rack before unmolding and cutting.