STEP 1:
In a heat safe bowl or a saucepan, melt the butter with the semi-sweet chocolate. Set them aside to cool while you get the rest of the ingredients together..
STEP 3:
In a separate bowl, use a whisk to combine the eggs, granulated sugar, brown sugar and vanilla until they're just barely combined; don't over-mix.
STEP 4:
Slowly pour in the butter and chocolate mixture to the eggs and sugar, while whisking. Use a sieve to sift the dry ingredients into the wet ingredients.
STEP 5:
Fold everything together until they're just barely combined. Pour the brownie batter into the prepared pan and bake at 325F for about 40 minutes.