Preheat oven to 400°F. Line a cookie sheet with parchment paper and set it aside.
Using a stand mixer or handheld electric mixer on high speed, cream together softened butter and sugar for about 2 minutes, until mixture is light and fluffy.
Add egg and vanilla to the bowl. Set the mixer to medium speed and beat mixture until smooth.
Add flour, baking powder, and salt to the bowl. With the mixer set on low speed, mix until a stiff dough forms. Add milk and continue mixing on low speed until thoroughly combined.
Optional: Divide dough into three equal sized sections. Leave one as is, then use gel coloring to dye one section blue and the other section red.
Use a cookie scoop or spoon to create tablespoon-sized balls of dough. Drop them onto the lined baking sheet. Alternatively, use a rolling pin to roll cookie dough into a ½-inch thick sheet, and cut out the shapes of your choosing.
Bake cookies in preheated oven for 9 minutes, or until the edges of each cookie are just barely golden brown.
Remove from oven and let cookies cool on the baking sheet for a couple minutes. Transfer to a wire rack to cool completely. When sugar cookies are cool, decorate as desired.