Preheat & Prepare Pan: Preheat your oven to 175 degrees C (350 degrees F). Line an 8*8 inch baking dish with parchment paper.
Make the Brownie Batter: In a bowl, mix the melted butter and sweetener together. Mix in the eggs and vanilla, add the almond flour, cocoa powder, baking powder and salt and combine.
Spread the Brownie Layer: Spread out the brownie batter into the prepared dish with a rubber spatula.
Make the Cheesecake Batter: In another bowl, cream cheese and erythritol and beat until smooth. Beat in the egg and vanilla extract, and beat again until creamy.
Swirl the Layers: Spoon cheesecake filling by dropfuls over brownie batter. Swirl slightly with a knife or skewer for a marbled effect.
Bake the Brownies: Bake 28 - 32 minutes, or until center is almost set. Do not overbake.
Cool and Chill: Let the brownies cool in the pan for 15 minutes, then transfer to the refrigerator to become cold, at least 1 hour, before slicing.