Start by preheating your oven to 350°F (175°C). Line your baking sheet with parchment paper. This helps prevent sticking and makes cleaning easier.

In a bowl, whisk together flour, baking soda, and salt. Set it aside. These dry ingredients will keep your cookies soft and fluffy.

In another bowl, beat butter, peanut butter, brown sugar, and white sugar. Mix until smooth and creamy. This step adds richness and flavor.

Now add the egg and vanilla extract. Mix well until everything combines nicely. The mixture should be smooth and light.

Slowly add the dry mixture into the wet mixture. Stir until just blended. Don’t overmix or the cookies will be tough.

Take half of the dough and move it to another bowl. Add cocoa powder and milk to this half. Mix it well to form your chocolate dough.

Spoon both doughs side by side on a clean surface. Gently swirl them together using your hands or a spoon. Don’t mix too much — you want a marbled look.

Scoop tablespoon-sized dough portions and place them on the baking sheet. Leave some space between each cookie so they can spread while baking.

Bake for 9–10 minutes, or until the edges look firm. The center should still be soft. They’ll continue to cook as they cool.

Let your cookies cool on the baking sheet for a few minutes. Then move them to a cooling rack. Enjoy them warm or at room temperature.