Crust: Whisk flour and cocoa in medium bowl. Beat butter and powdered sugar in another medium bowl until well blended; beat in egg yolk. Add flour mixture in 2 additions, beating just until blended. Gather dough into a ball; flatten into disk. Wrap in plastic wrap and refrigerate 2 hours.

Divide dough into 8 equal parts. Press each onto bottom and up sides of a 3½-inch diameter tartlet pan with removable bottom. Refrigerate crusts 1 hour or freeze 30 minutes.

Preheat oven to 375° F. Bake cold crusts until set and dry-looking, about 12 minutes, pressing with back of spoon if bubbles form. Cool crusts in pans. Note: May be made one day in advance; cover and store in pans at room temperature.

Mousse: Place milk chocolate in medium bowl. Bring 1½ cups whipping cream to simmer in small saucepan. Pour hot cream over chocolate; let stand 1 minute, then whisk until melted and smooth. Cover with plastic wrap and refrigerate until cold, at least 4 hours or overnight.

Caramel: Combine ½ cup whipping cream and ¼ cup unsalted butter in small saucepan; stir over medium heat until butter melts. Remove from heat.

Combine sugar and 3 Tablespoons water in a heavy small saucepan. Stir over medium-low heat until sugar dissolves. Increase heat and boil without stirring until color is deep amber, occasionally brushing down sides of pan with wet pastry brush and swirling pan, about 8 minutes. Immediately add hot cream-butter mixture (mixture will bubble vigorously). Remove from heat and stir until any caramel bits dissolve. Transfer caramel to small bowl and chill until slightly firm (semi-soft), stirring often, about 40 minutes.

Assembly: Spoon caramel into center of baked crusts (1½ to 2 Tablespoons for each crust). Set aside.

Using electric mixer, beat chilled chocolate-cream mixture until peaks form. Using a star tip, pipe on top of caramel in crusts, dividing equally (about 1/3 cup for each crust). Chill at least 2 hours. Note: May be made 8 hours in advance. Cover and keep refrigerated.

Remove tartlets from pans. Lightly sift cocoa powder over tartlets and serve. Yield: 8 tartlets.

Note: If using 4½-inch tartlet pans, divide dough into six equal parts. Add 2 heaping Tablespoons of caramel to each prepared tartlet. Pipe ½-cup mousse on top of caramel. Yield: 6 tartlets.