Preheat and prep the pan. Heat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving an overhang for easy lifting. Lightly grease the parchment.

Make the cream cheese swirl. In a bowl, beat 8 oz softened cream cheese until smooth. Mix in 1/3–1/2 cup powdered keto sweetener, 1 large egg yolk, 1 teaspoon vanilla, 1–2 teaspoons lemon zest, and 1 tablespoon fresh lemon juice. Beat until creamy with no lumps. Set aside.

Mix dry ingredients for the brownie base. In a separate bowl, whisk 1 1/2 cups almond flour, 2 tablespoons coconut flour, 1/2 cup granular keto sweetener, 1 teaspoon baking powder, 1/4 teaspoon salt, and 1–2 teaspoons lemon zest.

Add the wet ingredients. Whisk 2 large eggs, 6 tablespoons melted butter (cooled slightly), 1 teaspoon vanilla, and 1–2 tablespoons fresh lemon juice. Pour into the dry mix and stir until a thick batter forms. It should be spreadable but not runny.

Spread and swirl. Spoon the brownie batter into the pan and smooth the top. Dollop the cream cheese mixture over the surface. Use a knife or skewer to gently swirl the two layers together, making figure-eights. Don’t overmix.

Bake. Bake for 22–28 minutes, until the edges are set and lightly golden and the center still has a slight jiggle from the cream cheese. A toothpick should come out with a few moist crumbs, not wet batter.

Cool and set. Let the pan cool on a rack for at least 45 minutes. For the cleanest cuts, chill in the fridge for 1–2 hours before slicing.

Slice and serve. Lift the brownies out with the parchment and cut into 12–16 squares. Serve chilled or at room temperature.