preheat the oven to 350˚
butter a 9x13 and line it with parchment so you can lift it out as one big cookie, you can also use foil for this
cream the butter and sugar until soft and fluffy
add the eggs one at a time beating well after each addition
mix in the vanilla
pour the flour in on the butter
then use a fork to lightly mix in the baking powder and salt into the flour
then use the mixer to mix everything together until the dough comes together
scrape the bowl with a spatula to make sure everything is mixed in well
bake the cookie for 20 minutes at 350˚
when it's lightly browned remove it frm the oven and let it cool
when it's cool make the frosting by mixing the butter and powdered sugar together with a mixer, stir in the vanilla and salt
add just enough milk to make a light fluffy buttercream
frost the cookie and add sprinkles
lift the whole cookie out of the pan
cut into bars and serve