Preheat the oven to 350F/180C and butter the walls of an oven-safe ramekin using cold butter. Spread the butter until it is reduced to a single visible layer. Transfer the ramekin to the refrigerator while you prepare the batter.
Separate the egg, pouring the egg white into a stand mixer bowl and the yolk into a small mixing bowl. To the mixing bowl with the yolk, add the cream, vanilla, almond flour, cocoa powder, salt, and 1½ tsp stevia. Whisk to combine and set aside.
To the stand mixer, add the egg white, cream of tartar, and 1½ tsp stevia. Whisk until a stiff meringue forms. Using room-temperature egg white will result in achieving stiff peaks faster.
Use a spatula to fold the whipped egg white into the chocolate batter gently. Use a light hand while folding to avoid destroying any air bubbles. Removing the air bubbles will result in a flat soufflé.
Pour the batter into your prepared ramekin. Gently rotate the dish so the batter settles (no tapping). Then, use the tip of your thumb to create a ring around the souffle batter, as pictured.
Transfer the ramekin to a baking sheet and bake for 22-25 minutes. The soufflé should rise to a level just above the rim of the ramekin. Insert a toothpick in the middle of the soufflé to test if it is done. Serve warm.