Melt butter and chocolate pieces in double boiler. Stir until melted and smooth.
Add eggs to the chocolate mixture and whip until smooth. Add sugar and stir until smooth. Fold in the flour. Make sure to scrape the sides of the bowl to incorporate the flour.
Lightly butter the skillet. On low to medium heat, add chocolate batter to skillet. let cook until sides are firm and firmness starts to move toward the center. Carefully turn cake over to cook top. Remember to keep the cake on low to medium heat. The bottom will burn if the fire is too high.
When cake is done all around the outside, place on plate and top with powdered sugar.