Beat the butter, brown sugar, and 1/2 cup granulated sugar together in a large mixing bowl until well-combined, about 2-3 minutes.
Add peanut butter and mix until incorporated.
Mix in egg and vanilla extract until combined.
Add the flour, baking powder, and kosher salt. Mix in just until combined.
Scoop 1.5 tablespoons dough, roll in remaining 1/4 cup sugar.
Place the scoops of dough 2 inches apart on a baking sheet lined with parchment paper or a silicone baking mat.
Flatten cookies with a fork in a criss-cross pattern.
Bake cookies for about 8-10 minutes. The edges should look set and the middles should look soft and slightly underdone.
Let the cookies cool completely on baking sheet.
Notes
Butter: I use salted butter in this recipe. If you'd like to use unsalted butter, increase the salt to 3/4 teaspoon.
Peanut butter: I use regular peanut butter. I don't recommend using natural peanut butter in this recipe.
Flour: It's important to properly measure the flour. I recommend weighing the dry ingredient for the most accurate measurements. If not, you can stir or sift the flour to break it up, lightly spoon it into the measuring up, and then level. Avoid packing the flour as that will give you dry or dense cookies.
Kosher salt: I like to use kosher salt in this recipe. The kosher salt gives the cookies more of a salty bite throughout the cookie. If you would prefer to use table salt, use half of the amount.