Cream butter and granulated sugar with a mixer until pale and fluffy. Add yolks and vanilla, and beat for 1 minute. Reduce speed to low, and beat in flour and salt until combined. Cover, and refrigerate until firm, about 1 hour (or up to 1 day).
Roll dough into 1 inch balls and place 1 inch apart on a cookie sheet. Press finger or thumb into center of each dough ball to make an indentation.
Bake for 12-14 minutes then remove from oven.
Your indentations may have raised a bit while baking.
Press down on indents with a spoon to make them deep enough to hold your filling.
Remove cookies from cookie sheet and place on cooling rack.
Make caramel filling
Place unwrapped caramels and cream in a microwave safe bowl. Microwave on high for 30 second increments stirring between each until caramels and cream are mixed and smooth.
Using a spoon fill each cookie with caramel. Then sprinkle a small amount of sea salt on each.
Melt milk chocolate.
Place chocolate and shortening in a separate microwave safe bowl and microwave on 50 % power for 30 seconds. Remove and stir until smooth. If you feel as though you need to microwave more just add another 30 seconds. (If you overheat your chocolate if may seize so be careful. Once the chocolate is warm it should become smooth as you mix it.)
Drizzle melted chocolate over cookies and let set.