Combine butter and sugar in the bowl of a stand mixer (or in a large bowl and use an electric hand mixer) and beat until creamy and well-combined.

Add egg and vanilla extract and beat until completely combined.

In a separate, medium-sized bowl, whisk together flour, baking powder, and salt.

Gradually add dry ingredients into wet until completely combined.

Lay out a large piece of plastic wrap and transfer approximately half of the dough onto the wrap (dough will be quite sticky at this point, that's OK!).

Once dough has finished chilling, preheat oven to 350F (175C) and line a baking sheet with parchment paper. Set aside.

Generously dust a clean surface with flour and deposit one chilled cookie dough disk onto the surface. Lightly flour the dough and roll out to 1/8" (for thinner, crispier cookies) or 1/4" (for thicker, softer cookies). Continue to add flour as needed both on top of and beneath the dough so that it doesn't stick.

Allow cookies to cool completely on cookie sheet before moving and frosting.

Combine sugar, 2 Tablespoons of milk, corn syrup, and vanilla extract in a medium-sized bowl and stir until combined. If frosting is too thick, add more milk, about a teaspoon at a time, until the frosting is thick but pipeable. If you accidentally add too much milk, add powdered sugar until desired texture is reached.

If coloring the frosting, divide into bowls and color as desired at this point.

Pipe frosting on cookies and decorate with decorative candies, if desired.

Keep uneaten cookies sealed in an airtight container at room temperature.