Melt chocolate. Place chocolate in a heatproof bowl set over a saucepan with ½-inch of barely simmering water and stir occasionally until melted and smooth (or melt in the microwave on medium power in 20 second increments with frequent stirring). Set aside to cool slightly.

Make the cookie dough. Combine soft butter and sugar in a large bowl and mix well with a spatula until smooth and a bit fluffy-looking. Add egg and mix well until incorporated. It’s important that the egg is at room temperature or it will be difficult to incorporate since there isn’t so much butter in this recipe.

Add the slightly cooled melted chocolate and stir it in until evenly combined.

Sift flour, cocoa powder, baking powder and baking soda in a medium bowl. Add salt and whisk to blend evenly. Add half of the flour mixture into the chocolate mixture and fold it in until combined.

Add milk and fold it into the thick batter, then fold in the remaining flour mixture. Cover the bowl and refrigerate dough for 1-2 hours.

Make the peanut butter filling. Combine peanut butter and confectioner's sugar in a medium bowl and mix until smooth. It will form a thick paste. Use a teaspoon to scoop up the filing and roll each portion into smooth balls about 10g each. Place them on a parchment-lined baking sheet, plate, or tray. Repeat with remaining mixture until you have about 14 peanut butter balls. Loosely cover and freeze them for 30 minutes to an hour.

Preheat oven to 350°F. Line two large baking sheets with parchment paper.

When ready to bake, shape the dough balls. You can do this a couple of ways. The peanut butter filling softens easily at room temperature, so you can work with half of the batch at a time while the rest are in the freezer. Use a 1 ¼-oz cookie scoop to scoop up the chocolate cookie dough. With the dough still in the scoop, make an indentation in the center with your thumb, press a frozen peanut butter ball into the indentation, wrap the cookie dough around it to enclose it and then release the dough from the scoop. Roll it into a smooth ball. Now give it a marbled look by pinching the dough in half to expose the peanut butter filling, press these two halves together and then roll into a smooth ball. Repeat with remaining cookie dough and peanut butter ball.

Alternatively you can portion the dough, roll it into a smooth ball then flatten the dough ball out to about a 2 ½-inch round patty. Place a peanut butter ball into the center and then wrap the cookie dough around to fully engulf it. Roll it into a smooth ball and continue as above to create the marble effect.

Place dough balls 3 inches apart on the baking sheets and bake for 9-11 minutes or until the tops are shiny and the edges are set. The centers should still be a little soft and will set as the cookies cool.

Transfer baking sheets to a wire rack and let cookies cool for 2 minutes before transferring individually to a wire rack to finish cooling.