Start by preheating the oven to 350F, then line an 8x8 inch baking pan with parchment paper and set aside.
In a medium bowl melt the butter. Add in the brown sugar and whisk until well incorporated.
Add in the room temperature egg and vanilla and whisk again, to .mix everything together then add the peanut butter and stir to combine.
Add in the flour and salt and fold just until combined. Pour the peanut butter blondie batter into the lined baking pan, spread it evenly then bake for 20 minutes.
Once baked, pull out of the oven and let it cool for 5 minutes. While that's cooling, unwrap 16 Reese's chocolate peanut butter hearts.
Once the 5 minutes are done, press the Reese's heart to the top of the blondie evenly. Now you can either slice and enjoy right away or let the chocolate harden again and then slice for more of a clean finish.