Add the butter, white sugar, brown sugar, and vanilla to a bowl, then beat with a hand mixer until it looks creamy and combined. Add the egg and beat well until it is well-combined with everything else and the batter looks smooth.
In a separate bowl, whisk together the flour, sifted cocoa, and salt, then add this into the batter. Beat until a sticky dough forms. Using your hands, bring together the cookie dough then divide the dough in half. Shape each half into a disc shape. Wraps the discs with plastic wrap, then place into the fridge to chill for 2 hours.
Preheat the oven to 375°F. Prepare 2 to 4 baking sheets with parchment paper.
Remove one dough from the fridge and allow it to sit on the counter for 5 to 10 minutes to warm slightly. Unwrap the dough, and place between two pieces of parchment paper;
Roll the dough out, between the parchment, to be between 1/8” to 1/4” thick, then using a cookie cutter, cut out the cookies, then transfer them onto the baking sheets (12 per sheet).
Using a mini heart cookie cutter (or any small cookie cutter), cut out the centre of 1/2 of the cookies. Next, place the cookies into the freezer for 20 minutes.
Repeat steps on second disc of dough.
Remove the sheets from the freezer, then bake the cookies for about 10-12 minutes, or until they look matte.
Allow the cookies to cool to room temperature, then spread ½ of the cookies with peanut butter, then sandwich together with another cookie.