Preheat oven to 350F. Prepare a circular pan by lining the bottom with parchment paper and coating lightly with oil or nonstick spray.
Melt butter in a small saucepan over low heat. Add baking chocolate, cocoa powder, cream cheese, and heavy cream. Stir until chocolate is melted and ingredients are well combined, then remove from heat and set aside.
Combine eggs, sweetener, vanilla, and sugar-free syrup in a bowl. Add coconut flour and baking powder, and stir until there are no lumps.
Add chocolate mixture to the bowl and stir until well combined. Stir in nuts, if desired.
Pour about 2/3 of the brownie batter into the prepared pan and set aside.
Cheesecake
In a large bowl, cream together cream cheese, butter, greek yogurt, vanilla, and sweetener until well combined. Add sugar-free syrup, coconut flour, and protein powder. Mix until no lumps remain.
Add eggs one at a time, beating mixture until just combined. Stir in chocolate chips.
Layer the cheesecake mixture on top of the brownie batter in the pan. Spoon remaining 1/3 of brownie batter on top, and use a spoon or butter knife to swirl the layers together.
Bake in preheated oven for 35-45 minutes, or until edges begin to brown and center is almost set. Allow cheesecake to cool.
Chill in the refrigerator for at least 2 hours before serving.