preheat oven to 350 F. Prepare a 9-inch round cake pan with non-stick cooking spray. In a medium bowl, whisk together flour, salt and baking soda.
In a large bowl, beat softened butter and sugar together until fluffy. Beat in egg and vanilla extract until smooth. Add the flour mixture in two additions, beating after each addition until fully combined.
Gently fold in sprinkles and M&Ms with a spatula. Spread the cookie dough into the prepared cake pan. Bake for 30-35 minutes, until it is clean in the center. Let cool completely.
For the frosting, in a large bowl beat softened butter with powdered sugar, vanilla extract and milk. Add in more milk as needed to thin. Place the frosting into a piping bag and pipe frosting onto the cooled cookie cake.
