Preheat the oven to 350°F (175°C).

Grease a 13x9 inch baking pan and set aside.

Melt 4 ounces of unsweetened chocolate and 1 cup of butter together in a double-boiler over low heat.

Allow the melted chocolate mixture to cool slightly.

In a separate bowl, beat 4 eggs and 2 cups of sugar together until the mixture is pale yellow.

Add the cooled chocolate mixture to the egg and sugar mixture and beat until well combined.

In another bowl, whisk together 1 1/2 cups of flour, 1 tablespoon of instant espresso powder, 1 teaspoon of baking powder, and 1/2 teaspoon of salt.

Gradually add the dry ingredients to the wet ingredients and stir until just combined. Do not overmix.

Fold in 6 ounces of semi-sweet chocolate chips and 2 teaspoons of vanilla extract.

Pour the batter into the prepared baking pan.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs.

Allow the brownies to cool completely in the pan on a wire rack before cutting into squares.