Prep the pan. Heat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with a parchment sling so you can lift the cake out easily.

Mix the dry base. In a large bowl, whisk almond flour, coconut flour, baking powder, baking soda, salt, and sweetener until no lumps remain.

Whisk the wet ingredients. In a second bowl, whisk eggs, melted butter, sour cream, almond milk, and vanilla until smooth and creamy.

Combine to make the batter. Pour the wet mixture into the dry ingredients. Stir with a spatula until just combined. The batter should be thick but scoopable. If it feels stiff, add a splash more almond milk.

Split and flavor. Divide the batter into two bowls. In one bowl, leave it as vanilla. In the other, stir in cocoa powder and espresso powder. If the chocolate batter thickens, loosen it with 1–2 teaspoons almond milk.

Layer and swirl. Spoon alternating dollops of vanilla and chocolate batter into the pan, creating a checkerboard. Use a butter knife to gently swirl the batters in a figure-eight pattern. Don’t overmix—2–3 passes are enough.

Bake. Bake for 45–55 minutes, or until the top is set and a toothpick inserted near the center comes out mostly clean with a few moist crumbs. If it’s browning too fast, tent with foil for the last 10 minutes.

Cool completely. Let the cake cool in the pan for 15 minutes, then lift it out to a wire rack. Cool fully before slicing to keep the crumb intact.

Optional finish. Dust with a little powdered keto sweetener or drizzle with a light keto chocolate glaze if you want extra flair.