Heat oven to 325°F. Combine flour, cocoa, baking powder and salt in bowl; set aside.
Combine 1 cup butter, sugar and vanilla in large bowl. Beat at medium speed until creamy. Add egg yolk, continue beating until well mixed. Add flour mixture; beat at low speed until well mixed.
Shape dough into 1-inch balls. Place 2 inches apart onto ungreased cookie sheet. Make small indentation in center of each cookie with thumb or back of teaspoon.
Bake 10-11 minutes or until set. Cool 3 minutes on cookie sheet; remove to cooling rack. Cool completely.
Combine all filling ingredients except half & half in medium bowl. Beat at medium speed, scraping bowl often and gradually adding enough half & half for desired piping consistency.
Spoon filling into large disposable pastry bag fitted with large star tip. Pipe filling into indent in each cookie. Insert 5 sliced almonds sideways in a spiral pattern into filling.
- If cookies spread too much, refrigerate dough about 30 minutes.
- If you don’t have a pastry bag, spoon filling into large resealable plastic food bag. Snip off one corner of bag; pipe filling into center of each cookie.