Preheat oven to 425F and spray 4 cups of a muffin tin.
In a microwave-safe bowl, add the chocolate, butter, and heat on high power to melt, about 1 minute. Stop to whisk until smooth. If needed, return bowl to micro and heat in 15-second increments until chocolate can be whisked smooth.
Add the sugar, egg, egg yolk, and flour and whisk until smooth.
Pour batter into prepared muffin tins.
Bake for about 10 minutes, or until edges are set but center is still soft; don't over bake. Baking times will vary based on oven.
Allow cakes to cool in the muffin cups for about 1 minute, gently rim the edges of muffin cups with a knife, and slowly take it.
Top it with ice cream, hot fudge, and serve immediately. Cakes are best warm and fresh.