Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper.
In a medium bowl, whisk together the almond flour, cocoa powder, baking powder, and salt. In a separate large bowl, mix the melted butter and erythritol until smooth. Add the eggs and vanilla extract, whisking until fully incorporated. Gradually add the dry mixture to the wet ingredients, stirring until just combined. Fold in the sugar-free chocolate chips and walnuts if using.
In a small bowl, mix together 1/2 cup almond flour, 1/4 cup erythritol, 1/4 cup softened butter, 1 egg, and 1 teaspoon vanilla extract until smooth.
Pour half of the brownie batter into the prepared baking pan. Spread it evenly. Spread the cookie dough over the brownie layer, then pour the remaining brownie batter on top. Use a knife to swirl the two layers together.
Bake for 25-30 minutes. Check for doneness with a toothpick; it should come out mostly clean.
Let the brookies cool for at least 15 minutes before lifting them out of the pan and cutting into squares.