Preheat convection oven to 350°F (175°C) and line heavy-gauge baking sheets with silicone mats or parchment paper.

In a stand mixer fitted with a paddle attachment, cream the butter, granulated sugar, and brown sugar on medium-high speed for 3 minutes until aeration is achieved.

Add eggs one at a time, beating well after each addition, then incorporate the vanilla extract.

In a separate vessel, sift together the all-purpose flour, baking soda, and salt.

Reduce mixer speed to low and gradually add the dry ingredients to the creamed mixture until just combined; do not overwork the gluten.

Divide the base dough into two equal portions by weight.

In the first portion, fold in the cocoa powder until the color is uniform.

In the second portion, fold in the creamy peanut butter until fully integrated.

Using a 1-tablespoon scoop, take a portion of each dough and press them together, rotating slightly to create a marbled swirl effect without fully blending the colors.

Place the 2-ounce dough spheres on the prepared baking sheets, maintaining a 2-inch clearance between each unit.

Bake for 10 to 12 minutes until the edges are set and the surface loses its raw sheen.

Cool on the baking sheet for 5 minutes to allow the internal structure to set before transferring to a wire cooling rack.