1. Make the Vanilla Base Dough
In a large mixing bowl, cream the softened butter and sugar together until light and fluffy. Beat in the egg and vanilla extract. Gradually mix in the flour and salt until a soft dough forms.

2. Divide and Flavor the Dough
Split the dough into two equal parts. To one half, add the cocoa powder and knead gently until evenly combined. If the chocolate dough feels too dry, add the tablespoon of milk.

3. Chill the Dough
Wrap each dough portion in plastic wrap and chill for 30 minutes to make it easier to handle.

4. Shape the Zebra Pattern
Roll each dough into a thin rectangle, approximately the same size. Layer the chocolate dough on top of the vanilla dough and press gently to adhere. Roll the layered dough into a log from the short end to create a spiral effect when sliced. Alternatively, slice and stack strips of dough for a sharper striped zebra pattern.

5. Slice and Bake
Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper. Using a sharp knife, slice the dough log into 1/4-inch rounds. Place cookies 2 inches apart on the sheet. Bake for 10–12 minutes or until edges are lightly golden.

6. Cool and Serve
Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.