Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper or silicone baking mats.

In a medium-sized bowl, whisk together the flour, baking soda, and salt. Set aside.

In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy (about 3 minutes).

Add the egg and vanilla extract to the butter-sugar mixture, mixing gently until fully incorporated.

Gradually add the dry flour mixture to the wet ingredients and mix just until combined.

Divide the dough into two bowls. In one bowl, add cocoa powder and milk, mixing until a uniform chocolate dough forms.

Take a tablespoon of the plain dough and roll it into a ball, then do the same with a tablespoon of the chocolate dough.

Gently press the two dough balls together, forming a two-toned ball.

Repeat until you have about 12 to 15 cookie dough balls on the baking sheets, ensuring enough space for expansion.

Bake for 8 to 10 minutes, or until the cookies are set but still soft in the center.

Allow the cookies to cool on the sheets for 5 minutes before transferring to a wire rack to cool completely.