Cream butter and sugar until fluffy in a large bowl using a hand or a stand mixer, about 1-2 minutes.
Mix in egg yolks, baking powder, salt and vanilla extract until smooth, then mix in flour until dough forms.
Divide dough into four equal sections.
If adding mix-ins, add them now and mix until incorporated. If not adding mix-ins, go to the next step.
Roll each into a log that is about 6-7-inches long and approximately 1 ½-inches in diameter. (You can make these any size, larger or smaller, if you prefer.)
If coating the logs, place coatings on a large flat surface. Roll each log carefully in sugar and press lightly to adhere it, then wrap the logs in plastic wrap.
Chill at least 2 hours before slicing, or up to overnight. Tip: Place the logs standing up in drinking glasses so they stay circular.
When ready to bake, preheat oven to 350°F. Slice cookies desired thickness, no more than ¼-inches. Place on cookies sheets covered with parchment paper or silicone baking mats.
Bake for 16-18 minutes, or until they’re light golden around the edges and no longer glossy. You may want to check part way through baking to see if you need to rotate your pans in your oven, even if you normally don’t. These are delicate cookies and may brown in different areas quicker or slower.
Cool on cookie sheets then store in an airtight container for up to 5 days. Freeze for up to 3 months.