Preheat the oven:
Preheat your oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper or grease it with nonstick spray for easy removal.
Mix the wet ingredients:
In a large mixing bowl, cream together the softened butter, peanut butter, granulated sugar, and brown sugar until light and fluffy (about 2-3 minutes) using an electric mixer or whisk.
Add the egg and vanilla extract, and mix until well combined.
Combine the dry ingredients:
In a separate bowl, whisk together the flour, baking soda, and salt.
Gradually add the dry ingredients to the peanut butter mixture, stirring until just combined. Avoid overmixing.
Spread the dough:
Press the peanut butter dough evenly into the prepared baking pan using a spatula or your hands to create a smooth, even layer.
Bake:
Bake in the preheated oven for 20-25 minutes, or until the edges are lightly golden and a toothpick inserted in the center comes out clean.
Add the chocolate kisses:
As soon as the bars come out of the oven, immediately press the unwrapped chocolate kisses into the top of the warm peanut butter bars, spacing them evenly. The warmth of the bars will slightly soften the chocolate.
Cool and slice:
Allow the bars to cool completely in the pan before lifting them out using the parchment paper. Slice into 16 squares, making sure each square has a chocolate kiss on top.