To make the cookies: To the bowl of a stand-up mixer, add the butter, sugar and salt. Beat until light and fluffy, about 3 minutes. Add the egg, egg yolk, vanilla extract and beat once more until combined. In two batches, add the flour, being sure each addition is combined before adding the next one. Divide the dough in half, transfer it to two sheets of plastic wrap (it’ll be quite soft!), shape into a disc and transfer to the fridge. Allow to chill for about 1 hour. Ideally it should be at least 3 hours.
Line two baking sheets with parchment paper and set aside. Remove the first disc of dough from the fridge. Allow to come to room temperature for about 10 minutes. Liberally flour a rolling pin and work surface. Roll out the cookie dough to a 1/8-inch thickness. Stamp out the cookies with a cookie cutters. Transfer the cookies to the baking sheet, spacing them about 1-inch apart (these spread slightly). Transfer to the freezer for 10 minutes. Preheat the oven to 375 degrees F. Repeat with the second disc of dough.
Bake the cookies, one baking sheet at a time, for about 8 to 10 minutes, until lightly golden brown around the edges. Cool on the baking sheet before transferring to a cooling rack. Repeat with the remaining cookies.
To make frosting #1: whisk the powdered sugar and heavy cream together. It should be thick but pourable.
To make frosting #2: whisk the powdered sugar, heavy cream and food coloring together. Adjust the food coloring according to your liking. I was comfortable at one drop!
Dip the cookies, face side down into the frosting, turn them over and place them on a cooling rack or piece of parchment. I like to work in batches of three or four and then add sprinkles. Repeat until you've worked your way through all of the cookies. Allow to sit at room temperature for about 1 hour to set.