Step 1In a medium bowl, whisk the milk and vanilla into the powdered sugar, then whisk in the melted butter, to form a glaze. Cover and set aside until ready to coat the cookies.
Step 1Heat the oven to 350 degrees. Grease a13-by-9-inch baking pan. Line the bottom of the pan with parchment, and grease the parchment.
Step 2In a medium bowl, whisk together the flour, baking powder and salt. Set aside.
Step 3In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, cream together the butter and sugar until light and fluffy, 3 to 5 minutes. Slowly beat in the flour mixture until fully combined, then stir in the chocolate chips.
Step 4Press the dough into the prepared pan and bake until the cookie just begins to brown, 20 to 25 minutes (the cookie should be soft even as it cools). Rotate the pan halfway through for even baking.
Step 5While the cookie is still warm, pour the glaze over and tilt the pan so the glaze spreads evenly. Cut the cookie while warm into bars.