Cream softened butter and granulated sugar with an electric mixer until light and fluffy for 2-3 minutes.

Beat in eggs one at a time, then mix in vanilla extract until well combined, scraping the bowl as needed.

Whisk together flour, baking powder, and salt in a separate medium bowl.

Gradually add dry ingredients to wet ingredients, mixing on low speed until just combined without overmixing.

Divide dough in half, flatten each into a disk, wrap tightly, and refrigerate for at least 1 hour to prevent spreading.

Preheat oven to 375°F (190°C) and line baking sheets with parchment paper.

Roll out dough on a lightly floured surface to ¼-inch thickness and cut into shapes using cookie cutters.

Transfer cut cookies to prepared baking sheets, leaving 1 inch between them, and bake for 8-10 minutes until edges are lightly golden.

Cool cookies on the baking sheet for 5 minutes, then move to a wire rack to cool completely before decorating.

Beat softened butter on medium speed for 2 minutes until creamy.

Gradually add sifted powdered sugar, one cup at a time, mixing on low then increasing to medium until fluffy.

Beat in milk (or cream) and vanilla extract, adding a pinch of salt if desired; beat for 3-5 minutes until light and fluffy, adjusting thickness with more milk or powdered sugar.

If coloring, divide buttercream, add a tiny amount of gel food coloring to each bowl, and mix until desired colors are achieved.

Scoop buttercream onto the center of a cooled cookie and spread with an offset spatula or spoon, making swirls; add sprinkles immediately if desired.

Fit a piping bag with a round or star tip and fill with buttercream; outline the cookie edge, then fill the center with piped lines.

Optionally, smooth the filled area with an offset spatula or damp paintbrush and add sprinkles while the frosting is wet.

Use various piping tips (leaf, petal, small star) to create designs like rosettes, leaves, and dots by experimenting with pressure and angles for unique textures.