Chocolate Thumbprints

1 cup (1/2 lb.) butter, at room temperature
1/2 cup granulated sugar
1/4 cup firmly packed brown sugar
2 large egg yolks
1 teaspoon vanilla
2 cups all-purpose flour
1/2 teaspoon baking powder
1/4 teaspoon salt

5 oz chocolate chips

2 tbsp heavy cream

1 tbsp butter

1. In a bowl, with an electric mixer on high speed, beat butter, granulated sugar, and brown sugar until smooth. Beat in egg yolks and vanilla until well blended, scraping sides of bowl as needed.
2. In another bowl, mix flour, baking powder, and salt. With mixer on low speed, beat flour mixture into butter mixture until well blended. Cover bowl with plastic wrap and chill until dough is firm but pliable, about 30 minutes.
3. Shape dough into 1-inch balls and place 1 inch apart on 12- by 15-inch baking sheets. Press your thumb into the center of each cookie to make a 1/2-inch-deep indentation.
4. Bake cookies in a 325° oven until lightly browned, 10 to 12 minutes; if baking more than one pan at a time, switch pan positions halfway through baking. Transfer cookies to a rack to cool completely (if indentations have disappeared, make them again while cookies are warm).
5. Carefully fill each indentation with about 1 teaspoon chocolate ganache. Let stand until ganache is shiny and firm to the touch, about 1 hour (or chill for about 30 minutes).

Chocolate ganache. In a microwave safe bowl melt chocolate, cream and butter - stir occasionaly, and let cool until thick but not firm, about 15 minutes. Makes about 1 1/4 cups.