Melt the unsweetened chocolate and butter on stove or in microwave until almost melted. Stir until completely melted. Set aside to cool for a few minutes.
In a large mixing bowl, beat egg until light yellow, 2-3 minutes. Add sugar and beat until mixed.
Add vanilla and chocolate mixture to the egg and sugar and mix until smooth. Add the flour and baking soda and stir until just combined. Pour into cast iron skillet. Rinse bowl to use for cookie.
Bake brownie for 10 minutes then remove from oven.