Preheat oven to 375ºF. Line 2 baking sheets with parchment paper.
In a large bowl, cream butter and sugar with an electric mixer until light and fluffy. Beat in egg, egg yolk and vanilla. Set aside. In a medium bowl, whisk together flour, baking powder, baking soda, cocoa powder and salt. Add dry ingredients to butter mixture and beat until just combined. Stir in chocolate chips. Drop 1-Tbsp. sized portions of dough, 2-inches apart, onto lined baking sheets. Bake at 375°F for 9 to 10 minutes or until edges are firm but centers are still soft. Cool cookies for 5 minutes on baking sheets and cool completely on a wire rack.
Store cookies in an airtight container at room temperature for up to 5 days.
Makes about 30 cookies
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