In a bowl whip butter with sugars until creamy and fluffy. It can take up to 10 minutes. I recommend using a stand mixer.
Add eggs, vanilla, and yolk. Whip on high speed until the mixture doubles its size and gets very creamy.
Make sure you are using a dough hook for this step. Add flour, cornstarch, baking powder and baking soda. Mix on slow speed until every ingredient gets in. The dough should be very soft and sticky.
Chop up the chocolate into large pieces. Add them to the dough and mix for another 2 minutes.
Scoop the dough into the skillet or pie pan and spread it evenly. I threw some extra chocolate in the middle of the skillet for extra melting chocolate. Put some dough over the extra chocolate. Sprinkle the top with chocolate drops. Bake for 15-25 minutes. Take it out of the oven when it gets brownish. Let cool.
Serve warm with a scoop of ice cream. I recommend to sprinkling with extra sea salt.
