Preheat oven to 325 degrees Celsius and line a loaf pan with parchment paper.
Mix together the sugar, butter, and flour for the dough all in one bowl, whisking and folding until well combined for a few minutes.
Transfer the cookie dough to your lined loaf pan and press into pan. I typically use the bottom of a glass cup to press it down, or my hands!
Bake for 25-28 minutes. Let this cool completely before slicing into 16 bars and piping on the buttercream!
For the buttercream combine all ingredients and whisk on high speed until combined. If too thick, add a splash of milk. If too thin, add a tablespoon of powdered sugar. Pipe onto the cookie bars and enjoy!
Makes about 16 squares. Keeps for 5-7 days, best if refrigerated.
