Refrigerate or freeze 2 tablespoons of the sugar-free nutella spread until firm. (You can do this ahead of time in the fridge. If using the freezer, you can do it closer to last minute since it’s faster, but don’t keep it in there so long that it’s completely solid.)
Mix the almond flour, coconut flour, erythritol, cocoa powder, and gluten-free baking powder in a mug.
Stir in the egg and the remaining tablespoon of sugar-free nutella spread, until the batter is smooth.
Make a large, deep well in the center of the batter, holding it open with two spoons, and scoop the hardened sugar-free nutella spread (from the fridge) into it.
Microwave on high for about 40-50 seconds, until the cake is just barely set but the center is still gooey. (Do not overheat, otherwise the center will not be like “molten lava”.)