Preheat oven to 175°C/350°F. Line a 9×9 inch pan with parchment.
In a bowl, whisk melted butter with 100 g erythritol. Add 3 eggs, whisking until smooth, then stir in vanilla.
In another bowl, combine almond flour, cocoa, baking powder, and salt. Fold dry into wet until combined; stir in chocolate chips. Batter should be thick but spreadable.
In a small bowl, beat cream cheese until smooth. Add 1 egg, 60 g erythritol, vanilla, and lemon zest; beat into a silky mixture.
Spread brownie batter into pan evenly. Drop spoonfuls of cheesecake mixture on top and swirl with a knife.
Bake 22–30 minutes until edges set and center has slight jiggle. Cool 30 minutes, then chill 2 hours for best slicing.
Lift slab from pan using parchment, slice into 12 squares, and serve. Store chilled.