Preheat the oven to 350 degrees Fahrenheit and select a muffin/cupcake tray. Add the ground almonds, coconut flour, baking soda, cinnamon, cocoa powder, erythritol and salt to a large mixing bowl. Stir together well to combine. You may use an alternative low carb sweetener if desired, adjusting quantities to taste.
Add the lemon juice, almond milk, melted butter, and vanilla extract to the bowl with the dry ingredients. Melt the peanut butter briefly in a microwave or on the stove to soften a little. Add the softened peanut butter to the bowl. Beat everything together well to combine. The mixture will be thick and dry at this stage.
Crack the eggs into a separate bowl. Whisk the eggs together well to combine. Beat the eggs for a minute or two until light and frothy.
Add the beaten egg mixture to the flour mixture. Beat everything together until smooth and well combined. You should have a thick muffin batter.
Arrange 9 muffin cases in a muffin/cupcake tray. Divide the muffin batter equally between the 9 cupcake cases. Transfer the tray to the oven and bake for 20-22 minutes or until the muffins are cooked through and golden brown all over. Set the cakes aside to cool down a little before serving.