Preheat your oven to 350°F and line a 9x13-inch baking dish with parchment paper. In a mixing bowl, cream together the softened butter and brown sugar until the texture becomes light and fluffy, then incorporate 1 teaspoon of vanilla. Add in the flour, cocoa powder, baking powder, salt, instant coffee crystals, and espresso powder, stirring until the mixture is thoroughly blended. Gently fold in the chocolate chips. Press the batter evenly into the lined baking dish. Place in the oven and bake for 18 to 20 minutes. Meanwhile, prepare the topping by warming the condensed milk and 1 tablespoon of butter in a small saucepan over medium-low heat for 8 to 10 minutes, stirring constantly until the mixture becomes smooth and cohesive. Remove from heat and stir in the remaining 1 teaspoon of vanilla. Once the initial baking time is complete, pour the milk topping evenly over the surface of the cookie base. Return the dish to the oven and continue baking for another 5 to 8 minutes, or until the topping begins to bubble. Let the bars cool completely for at least 45 minutes before slicing. If desired, garnish with additional chocolate chips or a sprinkle of sea salt.