Prep your pan and oven. Heat the oven to 350°F (175°C). Grease an 8x8-inch square pan or a 9-inch round pan, then line with parchment for easy removal.

Mix the dry base. In a large bowl, whisk almond flour, coconut flour, baking powder, and salt until evenly combined and lump-free.

Whisk the wet ingredients. In another bowl, whisk sweetener (1/2 cup), eggs, melted butter, almond milk, vanilla, and sour cream until smooth.

Combine to make the batter. Add the wet mix to the dry ingredients and stir until just combined. The batter should be thick but spreadable. Let it sit 2–3 minutes to hydrate.

Make the cinnamon swirl. In a small bowl, mix remaining 1/4 cup sweetener with 2 teaspoons cinnamon. This will be your swirl layer.

Layer the batter. Spread half the batter into the pan. Sprinkle the entire cinnamon-sweetener mixture evenly over it. Dollop the remaining batter on top and gently spread to cover.

Optional crumb topping. Toss almond flour, sweetener, and cinnamon with cold butter using a fork until coarse crumbs form. Scatter over the top.

Bake. Bake 28–36 minutes, until the top is golden and a toothpick inserted in the center comes out mostly clean (a few moist crumbs are fine). Avoid overbaking or it can turn dry.

Cool. Let the cake cool in the pan for 15 minutes, then lift out using the parchment and cool on a rack. This helps it set and slice cleanly.

Optional glaze. Whisk powdered sweetener with almond milk and vanilla until smooth. Drizzle over the cooled cake.