Heat oven to 375°F. In large bowl, beat 1/2 cup sugar, 3/4 cup butter, the vanilla and egg yolk until light and fluffy. Add flour and cocoa; beat until well combined.
Shape rounded teaspoonfuls of dough into 1-inch balls; place 1 inch apart on ungreased cookie sheets. With index finger, make indentation in center of each cookie.
Bake 7 to 9 minutes or until set. Immediately remove from cookie sheets. Cool completely, about 10 minutes.
Meanwhile, in medium bowl, beat all filling ingredients until smooth. Spoon about 1/2 teaspoon filling into center of each cooled cookie. Press cherry into filling.
In small saucepan, melt chocolate chips with oil over low heat, stirring constantly. Spoon or drizzle melted chocolate over cherry on each cookie. Let stand until chocolate is set.