Preheat your oven to 325°F (160°C). Lightly butter a 9-inch springform pan. Tip: If your pan tends to leak, place it on a baking sheet to catch any drips.
In a large bowl, whisk together the melted butter, sweetener, eggs, and vanilla extract until well combined. Then, add the almond flour, cocoa powder, baking powder, and salt, mixing until smooth.
If the batter is too thick, stir in 1 to 2 tablespoons of water until it reaches a thick but pourable consistency. Gently fold in the chopped walnuts or pecans, if you're using them.
Spread this delicious brownie batter evenly over the bottom of your prepared pan. Bake for 15 to 20 minutes, just until the edges are set but the center still feels a little soft. Let it cool in the pan for 15 to 20 minutes.
Reduce your oven temperature to 300°F (150°C).
While the brownie cools, in another large bowl, beat the softened cream cheese for about two minutes until it's super smooth. Beat in the sweetener until fully incorporated. Finally, add the eggs, heavy cream, and vanilla extract, beating just until everything is combined. Be careful not to overmix!
Pour the silky cheesecake filling over the slightly cooled brownie crust and spread it out evenly. Bake for 35 to 45 minutes, or until the edges are set but the center still has a slight jiggle when you gently shake the pan. Remove from the oven and let it cool completely.
Once cooled, carefully run a knife around the edges of the pan to loosen the cheesecake before removing the springform sides. Cover the cheesecake with plastic wrap and refrigerate for at least 3 hours (or even better, overnight!) to ensure it's perfectly set and ready to enjoy.