1. Heat oven to 350°F. Beat butter, sugar and vanilla until well blended. Add egg yolks, 1 at a time, beating well after each addition. Beat in flour, cocoa and salt, beating until blended.
2. Roll dough into 1-inch balls; roll balls in pecans. Place about 2 inches apart on ungreased cookie sheet. Press thumb in center of each ball.
3. Bake 18 to 20 minutes or until set. Remove from cookie sheet to wire rack. Cool completely. Spoon 1 rounded teaspoon filling into each indentation. Cool 1 hour. Makes about 7 dozen cookies.
CHOCOLATE FILLING: Combine 2 cups sugar, 1/2 cup HERSHEY'S Cocoa and 1/2 cup milk in heavy saucepan. Bring to boil, stirring constantly; boil 2 minutes. Remove from heat; pour into bowl. Add 1/2 cup (1 stick) butter and 1/2 teaspoon vanilla extract. Do not stir. Cool 10 minutes. Beat on medium speed 3 minutes or until mixture is slightly thickened.