In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In the bowl of a stand mixer fitted with a paddle attachment or with a handheld mixer, beat the peanut butter and butter together until smooth, then add the dark and granulated sugars. Beat for 3 minutes, until fluffy.

Add the eggs and yolk, one at a time, followed by the vanilla extract. Beat until combined.

Gradually fold the flour mixture, followed by chopped chocolate, into the peanut butter mixture.

On a parchment-lined tray, scoop ¼ cup (2oz/57g) sized portions of dough, then roll into a ball and place on the tray. Cover and transfer to the refrigerator for at least 1 hour (or up to 2 days) to chill thoroughly.

When ready to bake, preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper.

Place about 6 balls of dough 3 inches (7 ½cm) apart on the baking sheet and flatten slightly with your hand.

Bake for about 15 minutes, until the edges are just starting to brown and the cookies are baked through.

Transfer to a wire rack and let cool while you bake the remaining cookies.

Store completely cooled cookies in an airtight container at room temperature for up to 5 days.