Heat oven to 350 degrees F. Coat 13 x 9 x 2-inch baking pan with nonstick vegetable-oil cooking spray.
Sift together flour, cocoa powder, 2 tablespoons espresso powder, baking powder and salt into a bowl.
With electric mixer on medium-high speed, beat 1/2 cup butter in a large bowl until smooth and creamy. Add the brown sugar and granulated sugar and beat until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in the vanilla until blended.
On low speed, beat in the flour mixture until well blended. Stir in the pecans if using and 1 cup milk chocolate chips until evenly distributed. Spread the batter evenly into the prepared baking pan; the batter will be a bit sticky.
Bake in 350 degree F oven for 20 to 25 minutes or just until edges begin to pull away from sides of pan. Cool the cookie completely in the pan on a wire rack.
Stir together warm milk and espresso powder in a small bowl until the 1 tablespoon espresso powder is dissolved. Beat together the confectioners sugar, 1/2 cup softened butter and vanilla in a medium-size bowl until well blended and smooth. Gradually beat in warm espresso mixture until the frosting is smooth.
Once cookie bar in pan has cooled completely, spread evenly with mocha frosting. Sprinkle top with the 1/2 cup milk chocolate chips. Place pan in refrigerator briefly until frosting is set. Cut into 36 bars. Store in the refrigerator. Makes 3 dozen bars.
