Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper.
In a large bowl, whisk the melted butter and erythritol together. Add the eggs and vanilla extract and whisk until smooth.
Add the almond flour, cocoa powder, salt, and baking powder. Stir with a spatula until a thick, smooth brownie batter forms.
In a small bowl, stir together the peanut butter, powdered erythritol, and melted butter until smooth and slightly runny.
Pour the brownie batter into the prepared pan and spread it evenly. Drop spoonfuls of the peanut butter mixture on top, spacing them out across the surface.
Use a toothpick or the tip of a small spoon to drag through the peanut butter and chocolate, creating a swirl pattern. Do not over-swirl or the two will blend together.
Bake for 20 to 24 minutes, until the edges are set and the center is just barely firm. A toothpick should come out with moist crumbs.
Let the brownies cool completely in the pan before cutting into 16 squares. They taste even better after being refrigerated for an hour.
