Preheat your oven to 350°F (175°C) to ensure even baking.
Line a 9x9 inch baking pan with parchment paper for easy removal.
In a large mixing bowl, combine the melted butter and peanut butter, whisking until smooth. Add the egg, brown sugar, and vanilla, mixing until well combined.
Gently fold in the flour until just combined to keep the batter light and airy.
Pour the batter into the prepared pan and spread evenly. Bake for 20-25 minutes until the edges are golden brown and the center is set.
Let the bars cool for about 5 minutes before pressing the chocolate Kisses into the top.
Allow the bars to cool completely, then lift them out and cut into squares or rectangles.
