Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper or silicone baking mats.
In a medium bowl, whisk together the flour, baking soda, baking powder, and salt until evenly combined. Set aside.
In a large bowl, beat the softened butter and 1 1/2 cups sugar together with an electric mixer on medium speed for 2-3 minutes until light and fluffy.
Add the egg, sour cream, and vanilla extract to the butter mixture. Beat on low speed until fully incorporated and smooth, scraping down the sides of the bowl as needed.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed just until no flour streaks remain. Do not overmix or the cookies will be tough.
Scoop 2-tablespoon portions of dough and roll each into a smooth ball between your palms. Roll each ball in the extra granulated sugar to coat completely.
Place dough balls 3 inches apart on the prepared baking sheets. Gently press each ball down slightly with the bottom of a glass to flatten to about 1/2-inch thickness.
Bake for 10-12 minutes until the edges are set but the centers still look slightly underdone and puffy. They should barely have any color on top.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. They will firm up as they cool but remain soft throughout.
