Cream butter and sugar until fluffy in large bowl – stand/hand mixer.
Mix in yolk, baking powder, salt, vanilla until smooth.
Then add flour until dough forms.
Divide dough in half – roll each into a log (about 6-7 inches long, about 1 ½ inches in diameter).
Sprinkle sanding sugar on a flat surface, roll each log carefully.
Wrap each log in plastic wrap, store upright in fridge at least 2 hours.
Heat oven to 350℉. After refrigeration, slice to desired thickness (I prefer ¼ inch).
Place on parchment paper or baking sheets on cookie sheet.
Bake ~10 minutes, or until edges are golden brown (Grandma Joyce said 16-18 minutes, but my oven was way different and 10 was good for me).
Cool, eat up to 5 days, freeze 3 months.