Preheat oven to 350°F and line baking sheets with parchment paper or silicone mats.
In a mixer, cream together the butter, sugars until smooth. About 5 minutes
Add the egg, egg yolk, vanilla extract and birthday cake emulsion and mix until combined.
Stir in flour, baking soda, and salt just until combined. Be sure not to over mix.
Using a cookie scoop (mine holds 1.5 Tablespoons), scoop the dough and roll lightly into balls, then roll generously in sprinkles.
Place dough balls well-spaced on baking sheet, and smash down lightly, avoiding crushing the sprinkles. These cookies will spread.
To get the really even coating of sprinkles onto the tops of your cookies, gently lift up each flattened cookie. Put the cookie dough face down into the sprinkle bowl, then gently press down on it to squish even more sprinkles into the dough. Return the cookie to the cookie sheet, extra sprinkle side up.
Bake for 9–11 minutes, until edges are golden brown but centers look soft.
Cool completely on the pan for the best chewy texture.