Place butter in a large mixing bowl and beat for 1 minute until fluffy and pale. Add in the sugar and brown sugar and mix until well incorporated. Add in the egg and vanilla and mix to combine.
In a medium mixing bowl, stir together and flour, baking powder, and salt. Add the dry ingredients to the wet ingredients in 2 additions, mixing after each until just combined.
Lay a piece of waxed paper on a flat surface. Scoop the cookie dough onto the center of the paper. Use the paper to form the dough into a log about 2 inches in diameter.
Wrap the waxed paper around the cookie dough log and chill in the fridge for at least 1 hour (up to overnight - see note).
Preheat your oven to 350 degrees F. Line a baking sheet with parchment paper or a silicone baking mat.
Unroll the cookie dough and cut the log into 1/4-inch slices. (You should get about 36 cookies total.) Place the dough slices on the prepared baking sheet about 2-inches apart.
Bake for 10-14 minutes until the bottom edges are just starting to turn golden brown.
Remove from the oven and allow to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
Peppermint Bark
Add the peppermint extract in when you add the vanilla extract. Stir in the crushed peppermints and white chocolate chips once the cookie dough is mixed together. Proceed as directed.
Pistachio
Add in the almond extract instead of vanilla extract. Add the green food coloring once the dry ingredients have been mixed in. You can add more or less color to reach your desired shade. Stir in the chopped pistachios once the cookie dough is mixed together. Proceed as directed.
Toffee
Stir in the toffee bits once the cookie dough is mixed together. Proceed as directed.
Notes
Nutrition is for basic cookie dough with no mix-ins.
You can chill the cookie dough in a fridge for up to 5 days if wrapped in plastic wrap.
You can split the base dough in half and make two flavors from one batch.