Prep your pan and oven: Heat oven to 325°F (163°C). Line the bottom of an 8- or 9-inch springform pan with parchment and lightly grease the sides.
Make the crust: Stir almond flour, melted butter, sweetener, cinnamon, and salt until sandy and moist.
Press into an even layer across the bottom of the pan. Par-bake for 8–10 minutes until lightly set. Cool for 5 minutes.
Whisk the swirl: In a small bowl, mix melted butter, sweetener, cinnamon, and vanilla.
Set aside; it will thicken slightly.
Blend the filling base: In a mixing bowl, beat the cream cheese until smooth, about 1 minute. Add Greek yogurt, sweetener, vanilla, lemon juice, and salt. Beat until creamy with no lumps.
Add eggs and protein: Beat in eggs one at a time on low speed.
Sift in protein powder and mix on low just until incorporated. Do not overmix.
Assemble with swirl: Pour half the filling over the crust. Drizzle half the cinnamon swirl over the top and gently swirl with a skewer or knife.
Add remaining filling and repeat with the rest of the swirl.
Bake gently: Bake 35–45 minutes, until edges are set and the center has a slight jiggle the size of a quarter. If browning too quickly, tent loosely with foil.
Cool slowly: Turn off the oven, crack the door, and let the cheesecake sit for 30 minutes. Then cool to room temperature on a rack.
This helps prevent cracks.
Chill to set: Cover and refrigerate at least 6 hours, preferably overnight, for clean slices and the best texture.
Make the icing: Beat cream cheese, powdered sweetener, vanilla, and a pinch of salt. Add almond milk 1 teaspoon at a time until drizzly.
Finish and serve: Release the springform, peel away the parchment, and transfer to a plate. Drizzle icing in a crosshatch pattern.
Slice with a warm, clean knife.