In a large bowl, cream together the softened butter and granulated sugar using an electric mixer until light and fluffy.

Beat in the eggs one at a time, then stir in the vanilla extract until well combined.

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

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined to form a soft dough; avoid overmixing.

Divide the dough in half, wrap each in plastic, and refrigerate for at least 1 hour (or up to 2 days) until firm for easier handling.

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

On a lightly floured surface, roll out one dough portion to ¼-inch thickness, cut shapes with cookie cutters, and transfer to baking sheets, re-rolling scraps as needed.

Bake for 8-10 minutes, or until the edges are lightly golden.

Let cookies cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely before decorating.

In a medium bowl, whisk sifted powdered sugar with 2 tablespoons milk (or water) and vanilla/almond extract until smooth and spreadable; adjust consistency with more milk or powdered sugar.

Divide and tint icing with food coloring as desired, then spread an even layer over each cookie using a small offset spatula or spoon; let set for 15-20 minutes until no longer sticky.

While the first layer is slightly tacky, add decorative elements: use contrasting icing for simple designs, immediately apply sprinkles to wet icing, or create marbled effects by dragging a toothpick through dots of contrasting icing.

Allow all decorated cookies to dry completely for several hours or overnight to prevent smudging before stacking or storing.