Preheat the oven to 325F and line a 9-inch springform pan with parchment paper on the bottom. Melt the butter and dark chocolate together in a bowl over simmering water, stirring until smooth. Remove from heat and let cool for 5 minutes.
Whisk in the powdered erythritol, then the eggs one at a time. Fold in the almond flour, cocoa powder, and salt. Spread evenly into the prepared pan.
Bake the brownie layer for 12 minutes until just set on top. Remove from the oven and let cool for 10 minutes while you make the cheesecake filling. Reduce the oven to 300F.
Beat the cream cheese and powdered erythritol until very smooth, about 3 minutes. Add the eggs one at a time, mixing on low speed after each. Mix in the sour cream and vanilla until just combined.
Pour the cheesecake mixture over the par-baked brownie base. Bake at 300F for 45 to 50 minutes until the edges are set and the center still has a gentle jiggle. Turn off the oven, crack the door, and let cool inside for 45 minutes. Then refrigerate for at least 5 hours.
Make the salted caramel by melting the butter over medium heat. Add the brown erythritol and stir for 2 minutes. Add the heavy cream and cook, stirring constantly, for 3 to 4 minutes until thickened. Remove from heat, stir in the vanilla, and let cool to room temperature.
Drizzle the salted caramel over the chilled cheesecake and sprinkle with flaky sea salt. Refrigerate for 15 minutes to set the caramel before slicing and serving.
