Preheat the oven to 350 degrees F (175 degrees C). Grease a 12-cup muffin tin.

Combine flour, sugar, cocoa powder, baking soda, baking powder, xanthan gum, and salt together in a bowl. Fold in chocolate chips.

Combine buttermilk, yogurt, oil, eggs, vinegar, and vanilla extract in a separate bowl. Add the flour mixture and mix well. Batter should be thick; scoop into the prepared muffin cups using a tablespoon until about 3/4-full. Sprinkle sliced almonds on top.

Bake in the preheated oven until tops spring back when lightly pressed and centers are still moist and gooey, 17 to 20 minutes. Cool in the tin for 5 minutes. Transfer to a wire rack to cool completely.