Preheat your oven to 375 degrees.
In a medium sized bowl combine your flour, cocoa powder, baking powder, and salt. Set aside.
In a large bowl, cream together your butter and sugar mixing for about 3 minutes until light and fluffy.
Add in your egg, milk and vanilla and mix till combined.
Whiling mixing, slowly add in your flour until everything is combined.
Press the dough into a greased 9x13 baking dish and spread out evenly with your hands or a rubber spatula.
Bake for about 15 minutes. Allow to cool completely.
Mix together all your ingredients for your frosting till creamy. Add in more milk or more powdered sugar to change consistency if needed.
Spread frosting evenly over your cooled cookie bars.
Melt chocolate chips in a small microwavable bowl in 20 second increments, stirring in between till smooth.
Scoop into a small ziplock bag and clip a teeny bit off the corner of the bag and drizzle chocolate over the top of the bars. Allow chocolate to harden.
Cut into squares and enjoy with milk!