Preheat the oven to 325 F and grease 2 mini-loaf pans. Line bottom and long sides of pans with parchment paper, and grease paper.
In a medium, microwave-safe dish, melt butter. Stir in cocoa powder until smooth. Set aside.
In a medium bowl, whisk together almond flour, erythritol, baking powder, baking soda, xanthan gum and salt.
In a larger bowl, beat eggs with electric mixer until frothy. Beat in ricotta, almond extract and stevia until smooth. Add dry ingredients and beat until thoroughly combined.
Place half of batter into chocolate mixture and mix until thoroughly combined. Alternating between light and dark mixtures, spoon dabs of batter into prepared pans until they are 2/3 full.
Place pans on a baking sheet and bake for 30-35 minutes or until top is set and a tester inserted in the middle comes out clean.