Preheat your oven to 325F. Line the bottom of a 9-inch springform pan with parchment paper and lightly grease the sides.
In a large bowl, whisk together the melted butter and granulated erythritol. Add the 2 eggs and vanilla extract and whisk until smooth. Fold in the cocoa powder, almond flour, and salt until just combined. Spread the brownie batter evenly into the bottom of the prepared pan.
Bake the brownie layer for 15 minutes until it is just set on top. Remove from the oven and let it cool for 5 minutes while you prepare the cheesecake. Reduce the oven temperature to 300F.
Beat the softened cream cheese and powdered erythritol with an electric mixer on medium speed until completely smooth and fluffy, about 3 minutes. Scrape down the bowl as needed to eliminate any lumps.
Add the eggs one at a time, mixing on low speed after each addition until just incorporated. Mix in the sour cream, vanilla extract, and lemon juice until smooth. Do not overbeat.
Pour the cheesecake batter gently over the brownie layer and smooth the top. Tap the pan on the counter a few times to release air bubbles.
Bake at 300F for 45 to 50 minutes until the edges are set but the center still has a slight jiggle. Turn off the oven, crack the door open a few inches, and let the cheesecake cool inside for 1 hour.
Remove from the oven and refrigerate for at least 4 hours or overnight before releasing from the springform pan and slicing.
