In a large bowl using a handheld electric mixer or using a stand mixer fitted with the paddle attachment, cream butter and sugar on medium-high speed until light and fluffy (2-3 minutes).
Add egg yolk and vanilla extract and continue to beat until well blended.
Add mascarpone cheese and beat just until combined.
Switch to low speed and add the flour in thirds, mixing well after each addition.
Divide the cookie dough in two and roll each into a log about 2 inches in diameter (the dough will be sticky). Wrap in wax paper, plastic wrap, then freeze for at least 1 hour.
When ready, pre-heat oven to 350F and line a baking sheet with parchment paper.
Remove dough from freezer and slice into ¼-inch pieces.
Place cookies on prepared baking sheet with about an inch of space around each one and bake for 10-12 minutes or until the bottom edges start to turn a light golden brown.
Remove from oven and allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Optional: dip in melted chocolate, place flat on a parchment-lined baking sheet, and sprinkle with chopped nuts. Refrigerate until set.
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Notes
The total number of cookies depend on how big or thick you make your cookies. I can usually get 32 cookies from this recipe.
If you don't have wax paper, parchment paper will work too but I find wax paper is easier to use.
As you can see, I’m not the best roller so my cookies come out rectangular than circular, and that’s okay!
It's easy to over-bake these cookies so watch them carefully. You want them to stay soft and tender and not crispy.