Adapted from Thomas Keller's Ad Hoc at Home

3/4 cup flour
1 cup Dutch processed cocoa (I used Droste)
1/2 teaspoon kosher salt
3 sticks (3/4 pound) butter, cut into tablespoon portions
3 eggs
1 3/4 cups sugar
1/2 teaspoon vanilla paste (may substitute extract)
6 ounces 61% cacao chocolate, chopped (I used E. Guittard semisweet chocolate wafers)

Preheat oven to 350º.  Butter and flour 9 inch baking pan.  Set aside.

Sift together dry ingredients.  Set aside.

Melt 1 1/2 sticks of butter in microwave safe bowl.  Add the remaining butter and stir till butter mixture is creamy looking and comes to room temperature.

In stand mixer with paddle attachment, mix eggs and sugar till thick and pale, about 3 minutes on medium speed.  Mix in vanilla.  Add one third of dry ingredients and mix till just incorporated.  Continue alternating dry ingredients and butter, mixing only till combined.  Add the chocolate and gently mix in to batter.

Pour into prepared pan and smooth top.  Bake 40-45 minutes until toothpick inserted in the middle of the pan comes out with just a few moist crumbs.  Cool, cut and serve.