Preheat your oven to 350°F and line a baking sheet with parchment paper.
Start by making the chocolate chip cookie dough. In a large bowl, mix together the melted butter, brown sugar, and granulated sugar until smooth. Add the egg and mix until fully combined. Add the flour, baking soda, and salt, and mix until just combined. Fold in the chocolate chips.
Next, make the reverse chocolate chip cookie dough. In a separate bowl, mix together the melted butter, brown sugar, and granulated sugar until smooth. Add the egg and mix until fully combined. Add the flour, cocoa powder, baking soda, and salt, and mix until just combined. Fold in the chocolate chips.
Scoop about 20 g of each dough, which is about 1 heaping tablespoon each. Place the two portions side by side, gently press them together, and lightly roll into a ball just enough to hold the shape. Do not overmix or twist the dough so you keep a clear half and half look.
Place the dough balls onto the prepared baking sheet, leaving space between each one.
Bake for 9 to 11 minutes, until the edges are set and the centers are still soft.
Let the cookies rest on the baking sheet for 5 minutes, then transfer them to a cooling rack to cool completely.