Using a double broiling method, melt butter and chocolate on low heat, set aside to cool.
Using a balloon whisk, whisk sugar and vanilla into chocolate mixture.
Add eggs slowly, one at a time, and whisk with each egg addition. Avoid over mixing the batter.
Whisk in cocoa powder, salt, and baking powder.
Place mixture in a greased (oil works well), parchment paper lined sheet pan and bake at 325F for 10-15 minutes in steamy oven (you may want to place a pan of hot water into the oven while the cake is baking).
Insert toothpick, cake should come out semi-clean.
Cool on rack and wrap and refrigerate. Serves 8-12. Enjoy with berries and whipped cream.