10 tablespoons unsalted butter, room temperature
1/3 cup sugar
2 large egg yolks
3/4 cup all-purpose flour
1/3 cup unsweetened cocoa powder
7 ounces milk chocolate chips
7 ounces semisweet chocolate chips
1 cup whipping cream
In an electric mixer, cream butter and sugar in medium bowl. Beat in egg yolks. Add flour and cocoa. Mix just until combined. Form dough into ball; flatten into disk. Wrap in plastic; chill 1 hour.
Preheat oven to 350F. Roll out dough on floured surface to 11-inch round (or 13-inch if using an 11-inch tart pan). Transfer to 9-inch (or 11-inch) tart pan with removable bottom.
Fold in overhang; press to form double-thick sides. Line crust with foil; fill with dried beans or pie weights.
Bake until edges are set, about 20 minutes. Remove foil and beans. Bake until firm, about 10 minutes longer; cool.
Place chocolate in medium bowl. Bring cream to boil in small saucepan. Pour hot cream over chocolate; whisk until smooth. Cool slightly. Pour chocolate mixture into cooled crust. Cover and chill at least 3 hours and up to 1 day.