Blend butter, sugar and salt with an electric mixer on low speed. Mix in egg and vanilla. Beat in as much flour as possible; stir in remaining flour.
Cover; chill for one hour.
Shape dough into one-inch balls; place 2 inches apart on ungreased baking sheets.
Bake at 375 degrees for 8 to 10 minutes, until lightly golden around edges.
Remove from oven; make a thumbprint in each cookie with thumb. Cool.
For Filling: blend butter and extract. Gradually add powdered sugar and milk to a piping consistency. Pipe filling into centers; sprinkle with crushed candy.