In a medium mixing bowl, whisk together the flour, baking soda, baking powder, and salt.
In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar together on medium speed until light and fluffy, about 3 minutes. Add the egg and vanilla, beating until well combined, stopping to scrape down the bowl as needed.
With the mixer on low speed, add in the flour mixture and beat until combined, stopping to scrape down the bowl at least once. Scoop the dough into small balls (about 2 tablespoons). If desired, roll the balls in additional brown sugar or decorating sugar. Chill the cookie dough balls for 1 hour.
Preheat the oven to 350F. Line two large baking sheets with parchment paper.
Place cookies 2 inches apart on the baking sheets and bake, one sheet at a time, for 12 minutes or until just starting to turn brown on the edges and then the center doesn’t appear pale. Let cool for a few minutes on the pan then transfer to a wire rack to cool completely. Store in an airtight container for up to 1 week.
Notes
It’s best to use room temperature eggs so they mix easily into your cookie dough. If your eggs are cold, place them in a bowl of warm water for a few minutes. This method will help to quickly bring them to room temperature if you have forgotten to take them out of your fridge beforehand.
Just like the eggs, butter should be room temperature as well. To soften butter, simply set it out on your kitchen counter for about 1 to 2 hours. However, the amount of time required depends on the temperature of your kitchen. If you’re short on time, cut the butter into cubes and set them out, as they will come to room temperature faster than a block.
I highly recommend using a triggered cookie scoop to portion the cookie dough. A cookie scoop ensures that all the cookies are the same size and bake evenly.
To avoid dry or dense cookies, measuring your flour accurately is important. The most common mistake is adding too much flour to the batter. The easiest and most precise way to measure flour is by using a digital kitchen scale. If you don’t have one, fluff the flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off. It’s important to avoid packing the flour into the measuring cup.
Don’t rush creaming butter and sugar together until light and fluffy. Creaming the butter and sugar helps incorporate air into the dough, which will help naturally leaven your cookies and create a tender crumb. If you don’t cream the butter and sugar enough, your cookies may be dense.
Don’t skip chilling the cookie dough! Refrigerating the dough prevents the cookies from overspreading when they bake.
Careful not to overbake these brown sugar cookies, as they’ll continue to firm up as they cool and become dry and crumbly if they’re already firm when you take them out of the oven.