Preheat oven to 350°F. Line 2 baking sheets with parchment paper or silicone mats
Place 8 oz of the chocolate and the espresso powder (if using) in a large microwaveable bowl and microwave on HIGH 2 min., stirring after 1 min.; stir until chocolate is completely melted (If your microwave has a chocolate melting function, you can use that instead).
8 oz semi-sweet chocolate, 1 tsp espresso powder
Add sugar, butter, eggs and vanilla. Stir until well blended.
3/4 cup brown sugar, 2 eggs, 1/4 cup butter, 1 tsp vanilla
Add flour and baking powder; mix just until combined.
1/2 cup flour, 1 tsp baking powder
Stir in chopped chocolate and additional mix-ins until evenly distributed.
8 oz semi-sweet chocolate, 2 cups mix-ins
Drop rounded tablespoonsful of dough (or use a #40 cookie scoop), 2 inches apart, onto baking sheets. Bake about 10 min. or until cookies are puffed and shiny. Cool on baking sheets 1 min. Remove to wire racks; cool completely.
Notes
Larger Cookies: Drop dough by scant 1/4 cups, 3 inches apart, onto baking sheets. Bake in 350°F oven 13 to 14 min. or until cookies are puffed and shiny. Makes 18 large cookies or 18 servings, 1 cookie each.Bite-Size Cookies: Drop heaping teaspoons of dough, 1-1/2 inches apart, onto baking sheets. Bake in 350° oven 6 to 7 min. or until cookies are puffed and shiny. Makes 5-1/2 doz. bite-size cookies or 22 servings, 3 cookies each.Bar Cookies: Spread into greased foil-lined 13x9-inch pan. Bake in 350° oven 22 to 24 min. or until puffed and shiny. Cool completely in pan on wire rack. Cut into 24 bars. Makes 24 servings, 1 bar each.