Preheat your oven to 350°F and line your baking sheets with parchment paper. Sift the powdered sugar for the frosting to remove any lumps—this ensures a smooth, lump-free icing that pipes beautifully. Measure out all your ingredients so everything is ready when you need it. I always sift my flour too when making sugar cookies to achieve a more tender, delicate crumb.

In a large bowl, beat together the butter and sugar until the mixture is light and fluffy, about 2-3 minutes—this incorporates air into the dough for a lighter texture. Add the vanilla extract and mix until combined. In a separate bowl, whisk together the flour and salt. Add the dry ingredient mixture to the wet ingredients and mix until just combined into a cohesive dough. I like to use Kerrygold butter for its rich flavor, and I make sure not to overmix once the dry ingredients are added to keep the cookies tender.

Fold the 1/4 cup nonpareils into the dough from Step 2 until evenly distributed—I add them at this stage rather than during mixing to prevent the colors from bleeding into the dough. Roll the dough out between two sheets of parchment paper to about 1/4 inch thickness for even baking. Using your Mickey Mouse cookie cutter, cut out 36 shapes and transfer them to the prepared baking sheets, spacing them about an inch apart.

Bake the cookies for 8-10 minutes until the edges are just set but the centers still look slightly underdone—they'll continue to cook as they cool. Remove from the oven and let the cookies rest on the baking sheet for 5 minutes to firm up, then transfer them to a wire rack to cool completely. This brief resting period prevents the delicate cookies from breaking during transfer.

While the cookies cool, whisk together the powdered sugar from Step 1, milk, and vanilla extract until you reach a smooth, pourable consistency—adjust with a touch more milk if it's too thick. Transfer the icing to a piping bag with a fine tip. Once the cookies are completely cooled, drizzle the icing over each cookie in your desired pattern, then immediately sprinkle the 4 tablespoons of nonpareils over the wet icing so they adhere. Allow the icing to set completely before serving, about 15-30 minutes.