Preheat your oven to 350°F (175°C). Line two baking trays with parchment paper to prevent the cookies from sticking and to encourage even baking.
Place the softened butter and granulated sugar in a large mixing bowl. Beat for approximately 2–3 minutes until the mixture looks light and creamy. This step helps create a tender cookie texture.
Add the egg and vanilla extract to the butter mixture. Beat until everything is thoroughly combined, scraping down the sides of the bowl as needed.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add these dry ingredients to the butter mixture, stirring until a soft cookie dough forms. Avoid overmixing, as this can make the cookies firm.
Divide the prepared dough into two equal portions. Leave one portion plain for the vanilla cookies. Add the cocoa powder to the second portion and mix until the color is even. If the chocolate dough becomes too dry, add a little milk. If it becomes too soft, incorporate a small amount of flour.
Take a small piece of vanilla dough and a similar-sized piece of chocolate dough. Gently press them together and roll them into a ball. You can also twist the two doughs together for a marbled effect. Avoid mixing them completely, so the contrasting colors remain visible.
Place the shaped dough balls in the refrigerator for 30 minutes. Chilling helps the cookies maintain their shape and prevents excessive spreading during baking.
Place the chilled dough balls on the prepared baking trays, leaving approximately 2 inches (5 cm) between each cookie. Gently flatten the tops if you prefer slightly wider cookies.
Bake for 10–14 minutes, depending on the size of the cookies and your oven. The edges should be set while the centers remain slightly soft. Avoid overbaking, as the cookies will continue to firm up as they cool.
Allow the cookies to rest on the baking tray for 5 minutes before transferring them to a wire rack. Let them cool completely, then serve with your favorite beverage.