1 cup (2 sticks, 8 ounces) unsalted butter, melted
2 cups (15 ounces) light brown sugar, firmly packed
¾ cup (2¼ ounces) Dutch-process cocoa
1 teaspoon salt
1 teaspoon baking powder
1 teaspoon espresso powder
1 Tablespoon vanilla extract
4 large eggs
1½ cups (6¼ ounces) white whole wheat flour
2 cups (12 ounces) semisweet or bittersweet chocolate chips
1. Preheat oven to 350 degrees F. Grease a 9x13-inch pan.
2. In a large bowl combine the melted butter and brown sugar and stir to combine. Place the bowl over a pot of simmering water and heat to between 110 and 120 degrees F - hot but not bubbling. Remove from the heat.
3. Stir in the cocoa, salt, baking powder, espresso powder and vanilla. Add the eggs, stirring after each until smooth. Add the flour and chocolate chips, again stirring until smooth. Spread the batter in the prepared pan and smooth the top with a spatula.
4. Bake for about 30 minutes, or until a cake tester or sharp knife inserted into the center comes out with wet crumbs, but not raw batter. Remove from oven and cool completely on a rack.