Cream the butter and granulated sugar together in a stand mixer until light and fluffy. Beat in the egg yolks and vanilla. Add 1 1/2 cups of the flour and mix to just incorporate. Whisk together the remaining flour and the shaved chocolate. Mix that into the dough just until incorporated--the dough will be stiff and maybe a little crumbly.

Use your hands to clump the dough together and place on a piece of plastic wrap. Shape the dough into a log approximately 2 to 3 inches in diameter. Wrap tightly and refrigerate until firm, at least 2 hours.

When you are ready to bake your cookies, preheat the oven to 350 F. Slice the dough log into 1/4-inch thick slices and place on a parchment-lined baking sheet, 1-2 inches apart (the dough will not spread, but do make sure there is room for hot air to circulate between the cookies).

Bake for 12-15 minutes, until the cookies are firm and turning golden on the bottom. Let cool for 5-10 minutes on the cookie sheet, before tossing the warm cookies very gently in the powdered sugar and placing on a cooling rack.