In a large bowl cream together the butter and both sugars until fluffy.
Beat in the egg and vanilla
Add in the flour, baking powder and salt.
Mix until just combined.
The dough will be very crumbly.
Add the sprinkles and stir into the dough.
Divide the dough into 6 equal portions and form each portion into a ball. Just until it comes together.
Next press the sprinkles all over the top and sides of the cookies.
Refrigerate for at least 30 minutes.
Meanwhile, preheat the oven to 375 degrees.
Bake the cookies for 15-18 minutes you want the edges to just be golden brown, we want the cookies to stay chewy.
Cool 5 minutes and then transfer to a rack to cool completely.
Meanwhile, make the buttercream for topping.
Using a hand mixer or stand mixer whip the butter until fluffy.
Add in the powdered sugar and beat to combine.
Add a splash of milk or coffee creamer if the buttercream is too thick.
Pipe the buttercream around the top of the cookies.
Top with sprinkles.
Serve!