Butter the cups of three muffin tins or two 6-ounce ramekins placed on a baking sheet and dust with cocoa powder to make the molten chocolate cakes easy to release after baking.
In a small microwave safe bowl, melt butter and chocolate in a microwave for 20 second intervals, stirring after each until melted. If you prefer, you could melt the chocolate mixture over a double boiler instead.
Stir the whole egg and egg yolk into the small bowl with the melted chocolate and butter mixture until well combined.
Pour the simple batter evenly between the three prepared muffin tins (or two small 6 ounce ramekins).
Place in an oven preheated to 425 F for a bake time of 7-9 minutes (ramekins will take 11-13 minutes) until the edges are cooked and the molten center appears almost set.
Allow to cool for one minute and then slide a butter knife around the sides of the cakes for easy release.
Place a baking sheet flat over the top of the muffin tin. Using oven mitts, hold the two together, pick them up and flip them over. Then remove the muffin pan
Use a spatula and carefully move the gooey chocolate cakes to serving plates