Get Ready: Preheat your oven to 350°F (175°C). Lightly grease a 9x9 inch (23x23 cm) metal cake pan with butter or cooking spray. For easy cake removal, create a parchment paper "sling" by lining the pan with two overlapping strips that extend over the edges.
Make the Swirl: In a medium bowl, simply mix all the cinnamon swirl ingredients together until you have a thick mixture. Set this aside for a moment.
Mix Dry Cake Ingredients: Grab a large bowl and whisk together the almond flour, granulated sweetener, baking powder, baking soda, and salt.
Add Wet Cake Ingredients: Now, stir in the beaten eggs, Greek yogurt (or sour cream), and vanilla extract into your dry mixture. Mix just until everything is combined and smooth.
Assemble the Cake: Pour your lovely cake batter into the prepared pan, spreading it out evenly.
Create the Swirl: Spoon dollops of your thick cinnamon mixture evenly over the cake batter. Don't worry about perfection; as it bakes, the cake batter will rise around these dollops, creating a beautiful swirled pattern.
Bake to Perfection: Pop the cake into your preheated oven and bake for 30-35 minutes. It's done when it's golden brown and a toothpick inserted into the center comes out clean.
Cool Down: Let the cake cool in the pan for 15-30 minutes before gently lifting it out using your parchment paper sling.
Whip up the Icing (Optional): While the cake is cooling, whisk together the powdered sweetener, heavy cream, and vanilla extract in a small bowl. Gradually add water, ½ tablespoon at a time, until the icing is thin enough to drizzle smoothly.
Drizzle & Enjoy: Once the cake is fully cooled, drizzle the sweet icing generously over the top. Cut into 12 squares and serve!