Cream the butter and sugar until fluffy. Beat in the egg, vanilla, and sour cream.

Combine the flour, salt, baking powder, baking soda, and cornstarch in a bowl then whisk together.

Add the dry mixture to the wet and mix on low until just combined. Use your spatula to scrape down the bowl.

Shape dough into a disk then cover in plastic and chill for one hour.

Preheat oven to 375 degrees F. Roll the dough out to 1/4 inch then use a 2-inch circular cookie cutter to cut the cookies. You may use any size round cutter or biscuit cutter just know this will affect the size and amount of cookies. Reroll the scraps and cut the remaining cookies.

Transfer cookies to parchment-lined baking sheets and bake at 375 degrees F for 8 minutes. Remove with a baking spatula to a cooling rack. Cool completely.

While cookies are cooling make the icing. Use the vanilla icing and mix together with two drops of neon green food gel. Mix well.

Ice cookies with a butter knife or a small spatula. Top with sprinkles.

Put white icing into a piping bag with a round tip. Write on Grinch. Add sprinkles around the writing.