For the Frosting:
To a saucepan over medium heat whisk together the milk and flour. Constantly whisking, bring mixture to a boil and remove from heat. Transfer to a small bowl and press a piece of plastic wrap directly on the mixture. Refrigerate until chilled, about 1 hour.
In the bowl of a stand mixer fitted with the paddle attachment add the butter, sugar, vanilla and salt. Beat until light and fluffy, about 5 minutes.
Scrape down the sides of a bowl and return mixer to low speed. Slowly add the chilled milk mixture and beat until combined. Add the whipping cream (if using) and beat until fluffy, about 2 minutes.
Divide frosting amongst 3 small bowls and add desired amount of food colouring. Transfer to piping bags fitted with a star tip attachment. Set aside until ready to use.
For the cookies:
Preheat the oven to 350(f) degrees. Line two baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment cream butter and sugar until fluffy, about 3 minutes. Add the eggs one at a time, followed by the vanilla extract. Scrape down the sides of the bowl after each addition.
Sift in the flour, baking powder and salt. Turn the mixer to low speed and slowly add the milk. Turn mixture up to medium speed and beat until batter is smooth, about 60 seconds.
Chill batter for 15 minutes as the oven preheats. Using a small ice cream scoop for even circles, smooth the batter on the prepared baking sheets. Top with sprinkles.
Bake for 5 to 7 minutes, until the centre has puffed and the edges are golden brown.
If desired, use a small biscuit cutter to shape cookies into perfect circles. Discard edges. Let cookies cool completely before frosting.
When ready to frost, pipe a swirl of frosting on the non-sprinkle side of the cookie and top with a second cookie. Enjoy!