Cream butter and sugar together in a mixer fitted with a paddle attachment for 3 minutes, until white and fluffy. This can also be done with a hand mixer.
Add egg and vanilla bean paste, mixing until smooth. Scrape down the sides of the bowl to ensure even mixing.
Add flour, baking powder, and salt. Mix slowly (so the flour doesn't puff up) until just combined. Do not overmix!
Sprinkle countertop with flour. Gather the dough into a ball and place on the counter. Lightly sprinkle the top of the dough ball with flour.
Using a rolling pin, roll the dough out to be 1cm thick (about the same as the width of your pinky).
Cut out shapes with a cookie cutter, and place onto parchment or silicone lined baking sheets. Don't worry about how far apart they are - we are not baking yet!
Gather remaining dough scraps into a ball, adding in 1 tablespoon of milk. Knead by hand on the countertop until combined, then roll out and repeat!
Once all of the dough is used up, cover cookie sheets with aluminum foil or plastic wrap and refrigerate for at least an hour (or up to 2 days if fully sealed).
Preheat oven to 350°. Re-arrange cookies on the cookie sheets so they are a few centimeters apart. Bake for 13-15 minutes. The cookies are done when the edges are golden.
Allow cookies to cool completely before decorating.