Preheat oven to 350 degrees and line a 9 x 9 baking pan with parchment paper and set aside.
Melt butter in microwave for 45 seconds. Pour into mixing bowl with paddle attachment. Add both sugars and beat for 1 minute.
Add cocoa, espresso powder, baking powder, salt and vanilla. Mix for another minute.
Gradually add in flour and eggs, beating until well combined.
Fold in walnuts and chocolate chips.
Pour batter into prepared pan and bake for 30 minutes, or until a knife can pierce the middle and come out cleanly.
Remove from oven and let cool on countertop while you prepare ganache.
In a double boiler set up, allow water to come to a light boil. Then add butter, milk, and chocolate chips to bowl on top. Stir to melt and combine ingredients.
Once the ingredients have melted together, carefully take off the bowl from the double boiler. Scrape sides and pour mixture evenly on top of brownies. Smooth with spatula.
Refrigerate for 1 hr or until cooled completely. Enjoy with a scoop of ice cream or a cold glass of milk!