In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, cream together the softened unsalted butter and granulated sugar on medium speed until light and fluffy, about 3-4 minutes.

Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract. Scrape down the sides of the bowl as needed.

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

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix. The dough will be soft.

Divide the dough into two equal portions. Flatten each portion into a disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 hours, to allow the dough to firm up.

Preheat your oven to 375°F (190°C). Line two baking sheets with parchment paper.

On a lightly floured surface, roll out one disc of chilled dough to about ¼-inch thickness. Use your favorite cookie cutters to cut out shapes. Reroll scraps as needed, being careful not to overwork the dough.

Carefully transfer the cut-out cookies to the prepared baking sheets, leaving about 1 inch between them.

Bake for 8-10 minutes, or until the edges are very lightly golden and the centers are just set. Do not overbake if you want soft cookies.

Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Repeat with the remaining dough.

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened unsalted butter on medium speed until creamy and smooth, about 2 minutes.

Gradually add 3 cups of sifted powdered sugar, alternating with the milk/cream and vanilla extract. Start on low speed to prevent a cloud of sugar, then increase to medium-high.

Beat until the frosting is light, fluffy, and smooth, about 3-5 minutes. If the frosting is too thick, add more milk/cream, 1 teaspoon at a time. If it's too thin, add more powdered sugar, 1 tablespoon at a time, until desired consistency is reached. Add a pinch of salt if using.

(Optional) If desired, divide the frosting into separate bowls and tint with food coloring.

Once the cookies are completely cooled, use a spatula, knife, or piping bag to spread or pipe the vanilla buttercream frosting onto each cookie.

Decorate with sprinkles, if desired.

Allow the frosting to set slightly before serving or storing.