In a large bowl, whisk together the softened butter and sugar until smooth and creamy.

Add the egg yolk, vanilla extract, and sweetened condensed milk. Whisk until fully combined.

Add the flour, baking soda, and salt. Mix until a soft dough forms and no dry flour remains.

Divide the dough evenly into two bowls.

To one-half, add the milk powder and sweetened condensed milk. Mix until well incorporated.

In a small bowl, dissolve the espresso powder in the hot water. Add the coffee mixture to the remaining dough and mix until evenly combined.

Refrigerate both doughs for 15 minutes to make them easier to handle.

Using a cookie scoop, alternate small portions of the condensed milk dough and coffee dough, gently pressing them together to create a marbled effect. Scoop onto the prepared baking sheet, leaving space between each cookie.

Bake for 10–12 minutes or until the edges are lightly golden and the centers still look slightly underbaked.

Let the cookies cool completely on the baking sheet. They will continue to set as they cool.