To bring the eggs to room temperature, place them in a bowl of warm water for about 10 minutes.

In the bowl of a stand mixer fitted with the whisk attachment, beat the eggs and sugar on medium-high speed until the mixture turns light in color and becomes thick and foamy, a process that takes roughly 3 to 5 minutes.

In a microwave-safe bowl, combine the chocolate chips and butter. Microwave in 30-second bursts, stirring in between, until fully melted. Stir in the instant coffee or espresso powder if using, then let the mixture cool slightly.

Once the egg and sugar mixture has reached the desired thickness, mix in the vanilla extract.

Fold the cooled chocolate mixture into the egg and sugar combination, stirring until well incorporated. Add in the salt and baking powder. Sift the cocoa powder and flour into the bowl, then gently fold everything together until fully combined.

Let the dough rest at room temperature for 30 minutes to thicken, making it easier to scoop.

Preheat your oven to 350°F and line your baking sheet(s) with parchment paper.

Using a 1.5 tablespoon scoop, portion the dough onto the baking sheet, placing 8 cookies per sheet.

Drop about 1/2 teaspoon of peanut butter onto each cookie dough ball. With the small end of a chopstick or similar utensil, gently swirl the peanut butter into the dough, creating "C" and "S" patterns. Avoid over-swirling to retain some thick peanut butter pools.

Bake the cookies for 5-6 minutes, until the centers still appear slightly wet. Remove them from the oven before they are fully baked to maintain a fudgy texture. If the cookies look uneven, gently press the edges with the back of a spoon to shape them into circles.

Let the cookies cool on the baking sheet for 2 minutes before transferring them to a wire rack to cool completely. Repeat the process until all the dough is baked.

Store the cookies in a single layer inside an airtight container at room temperature. Avoid using foil, plastic wrap, or parchment paper to prevent the peanut butter from sticking.