Heat oven to 350. Lightly grease a 33 cm x 23 cm baking pan.
Mix flour, cocoa powder, espresso powder, baking powder and salt.
Heat butter in a medium saucepan over low heat until melted. Remove from heat. Stir in brown sugar and vanilla with a wooden spoon. Add eggs, one at a time, beating well after each.
Stir in flour mixture and nuts until blended. Spread batter evenly in the prepared pan. Bake for 20 to 25 minutes or until edges pull away from the sides of the pan. Cool on a wire rack.
For frosting: Melt butter and chocolate chips in a small saucepan over low heat (or microwave in a bowl on high for 1 to 1.5 minutes, stirring every 30 seconds.) Stir in milk, vanilla and confectioners' sugar until smooth. Refrigerate until cold, about 45 minutes. When cold, beat with a whisk until pale and stiff. Spread frosting on cooled bars. Let set about 1 hour before cutting bars.