Preheat & Prep: Preheat the oven to 350°F (180°C). Lightly grease five 6-ounce (170ml) ramekins or oven-safe mugs.
Mix Wet Ingredients: In a single medium mixing bowl, add the eggs, melted butter, peanut butter, and vanilla extract. Whisk together until smooth and well-combined.
Add Dry Ingredients: Add the almond flour, sweetener, cocoa powder, baking powder, instant espresso powder, and salt directly into the same bowl with the wet ingredients.
Combine: Using a spatula or whisk, mix until the batter is thick and just combined. Do not overmix.
Portion: Divide the batter evenly among the 5 prepared ramekins.
Top & Bake: Sprinkle the sugar-free chocolate chips evenly over the top of each cake. Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean.
Cool & Serve: Allow the mug cakes to cool for a few minutes, as they will firm up. Serve warm, optionally with a dollop of keto-friendly whipped cream or yogurt.
Alternative Microwave Method:Divide the batter into 5 microwave-safe mugs. Microwave each mug individually on HIGH for 60-90 seconds, or until the cake is just set in the center.