Ingredients

• 3/4 cup plus 2 tablespoons all purpose flour
• 2 tablespoons plus 2 teaspoons unsweetened cocoa powder (preferably Dutch process)
• 1/2 cup (1 stick) unsalted butter, room temperature
• 1/2 cup plus 2 tablespoons powdered sugar
• 1 large egg yolk
• 3 1/2 ounces high-quality milk chocolate (such as Lindt or Perugina) chopped
• 2 cups heavy whipping cream, divided
• 1/4 cup (1/2 stick) unsalted butter
• 1/2 cup plus 2 tablespoons sugar
• 3 tablespoons water
• Unsweetened cocoa powder (for dusting)

Instructions

1. Whisk flour and cocoa in medium bowl. Beat butter and powdered sugar in another medium bowl until well blended. Beat in yolk. Add flour mixture in 2 additions, beating just until blended. Gather dough into ball; flatten into disk. Wrap in plastic and refrigerate 2 hours.
2. Divide dough into 6 equal pieces. Press each onto bottom and up sides of 4 1/2-inch-diameter tartlet pan with removable bottom. Refrigerate crusts 1 hour or freeze 30 minutes.
3. 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. DO AHEAD: Can be made 1 day ahead. Cover; store at room temperature.
4. Make filling: Place milk chocolate in medium bowl. Bring 1 1/2 cups 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. Combine remaining 1/2 cup cream and butter in small saucepan and stir over medium heat until butter melts; remove from heat. Combine sugar and 3 tablespoons water in 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. Spoon caramel into center of baked crusts (about 2 generous tablespoons for each crust). Set aside. Using electric mixer, beat chilled milk chocolate-cream mixture until peaks form; spoon atop caramel in crusts, dividing equally (about 1/2 cup for each crust) and spreading evenly. Chill at least 2 hours. DO AHEAD: Can be made 8 hours ahead. Cover and keep refrigerated. Remove tartlets from pans. Lightly sift cocoa powder over tartlets and serve.
5. Place milk chocolate in medium bowl. Bring 1 1/2 cups 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.
6. Combine remaining 1/2 cup cream and butter in small saucepan and stir over medium heat until butter melts; remove from heat. Combine sugar and 3 tablespoons water in 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.
7. Spoon caramel into center of baked crusts (about 2 generous tablespoons for each crust). Set aside.
8. Using electric mixer, beat chilled milk chocolate-cream mixture until peaks form; spoon atop caramel in crusts, dividing equally (about 1/2 cup for each crust) and spreading evenly. Chill at least 2 hours. DO AHEAD: Can be made 8 hours ahead. Cover and keep refrigerated.
9. Remove tartlets from pans. Lightly sift cocoa powder over tartlets and serve.