Place the butter and brown sugar in a stand mixer fitted with a paddle attachment. Beat until fluffy, about 2 minutes. Add in the egg and vanilla. Beat until just combined.
Whisk together the flour, 1 tablespoon of the pumpkin spice mix, the baking powder and salt in a large bowl. Pour half of the dry ingredients into the butter mixture and beat on low speed until combined. Repeat with the remaining half of the dry ingredients until a thick dough comes together. Chill for at least 1 hour or overnight.
Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper.
Combine the granulated sugar with the remaining 1 teaspoon pumpkin spice mix in a small bowl.
Shape the cookie dough using a heaping tablespoon and roll between your palms to form a round ball. Repeat until all the dough is rolled into balls. Place the balls into the spiced sugar and toss until completely coated. Place onto the prepared baking sheets, leaving about 2 inches of space between each cookie. Use your palms or the tines of a fork to press the cookies into thick disks.
Bake until golden brown, 10 to 12 minutes.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)