Combine ¼ cup sugar with lavender in bowl of small food processor or spice grinder.
Process until lavender is finely ground.
Add sugar, lavender sugar, and butter to a large bowl.
Beat with an electric mixer until light and fluffy, about 2-3 minutes.
Beat in egg yolks, one at a time.
Mix in vanilla.
Add flour.
Mix only to blend.
Turn dough out onto a piece of parchment paper.
Roll into a log that is 9 inches long.
Roll up in parchment.
Refrigerate 2 hours, or overnight.
When ready to bake, heat oven to 350 °F.
Line 2 sheet pans with parchment paper. Set aside.
Remove dough from refrigerator.
Slice into ¼ inch rounds.
Place on prepared cookie sheets.
Bake for 12 minutes, until just starting to brown around the edges.