Blend butter, brown sugar, and 3 tsp vanilla.
Stir in flour, milk, and chopped chips.
Shape in 1/2-inch balls. Place on ungreased cookie sheet. Make depression in center of each cookie with thumb. Bake 10-12 minutes at 375F.
Roll in powdered sugar. (I usually skip this step)
Combine over hot water the rest of the chips and the shortening. Stir until melted. Remove from heat.
Stir in Karo, 2 tbs water, and the 2 tsp vanilla. Fill each cookie.
Makes about 4 dozen.