Thumb Print Cookie

Hi everyone, I know its been forever and I apologize. I have been super busy and I haven't had the time to sit on the computer for a long period of time. Between vacation, weddings and the whole family visiting, there has been so little time. I have had a wedding back to back in the last month too, so you can imagine. I haven't baked in a few weeks but can't wait to try new treats. This was my first try baking Thumb print cookie and I have to say they are one of my favorite type of cookies now. They are light and I love the option to use any type of filling to make them taste even yummier. The best part also is that they are easy and fast to make.

1/2 cup butter
1/4 cup brown sugar
1 egg, separated
1/2 tbsp. vanilla
1 cup flour
Jelly or preserve (any flavor)