Preheat your oven to 350°F (175°C). This ensures that your cookies bake evenly and achieve that perfect soft texture.
In a large mixing bowl, combine the softened unsalted butter, granulated sugar, and light brown sugar. Using an electric mixer, beat on medium speed until the mixture is light and fluffy. This should take about 2-3 minutes.
Add the large egg and vanilla extract to the butter-sugar mixture. Beat until well combined, scraping down the sides of the bowl as needed.
In a separate bowl, whisk together the cake flour, cornstarch, baking powder, baking soda, and a pinch of salt. Gradually add this dry mixture to the wet ingredients, mixing on low speed until just combined.
Gently fold in the Reese's Peanut Butter Chips with a spatula until evenly distributed throughout the dough.
Using a cookie scoop or tablespoon, portion out the dough onto a baking sheet lined with parchment paper, leaving space between each cookie as they will spread slightly during baking. Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden. The centers will look slightly underbaked but will firm up as they cool.
Once baked, remove the cookies from the oven and allow them to cool on the baking sheet for 5 minutes before transferring them to a cooling rack to cool completely.
In a mixing bowl, beat together the softened unsalted butter and creamy peanut butter until smooth. Gradually add the confectioners’ sugar and beat until well incorporated. If the frosting is too thick, add a splash of cream or milk until you reach the desired consistency.
Once the cookies are completely cooled, use a spatula to generously spread the peanut butter frosting on top of each cookie. If desired, sprinkle chocolate sprinkles on top for an extra touch of sweetness.