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1
To make the cookies, preheat the oven to 375 F and line a baking sheet with a silicone baking mat or parchment paper. Set aside.
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2
In a medium-sized bowl, sift together the flour, cornstarch, baking soda and salt. Set aside.
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3
In a separate bowl, cream together the butter and the sugar. Add in the vanilla extract and egg and beat until combined. Add in the flour mixture about 1 cup at a time, and combine completely after each addition. Fold in the sprinkles and stir once or twice to incorporate, no more than that or else the color will bleed into the dough.
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4
Scoop the cookie dough (try to make them the same size) onto the prepared baking sheet, setting the cookies about 1 inch apart. Bake for about 12 minutes, checking often. Remove them from the oven once the outside edges are just starting to turn a golden brown. Remove immediately from oven at this point and allow to cool on the pan for 5 minutes. Allow to finish cooling on a wire rack.
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5
To make the icing, combine butter and vegetable shortening in a large mixing bowl until creamy. Add vanilla extract and salt.
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6
Add the confectioners' sugar one cup at a time and incorporate completely after each addition. The icing will appear dry after adding all of the sugar. Add the meringue powder.
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7
Add milk and beat until light and fluffy, about 8 to 10 minutes. Scoop into piping bag affixed with desired tip and add water as necessary to make the icing easy to pipe.
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8
To assemble, pair up the cookies by size. Pipe or spoon icing onto the bottom of one cookie and sandwich together with the second, same size cookie.
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9
Cookies adapted from TastyKitchen member Julia Mestas who adapted it from Yammie's Noshery's Funfetti Whoopie Pies recipe.
