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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.
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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.
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3
Preheat oven to 375u00b0F. 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:
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4
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.
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5
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.
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6
Spoon caramel into center of baked crusts (about 2 generous tablespoons for each crust). Set aside.
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7
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:
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8
Remove tartlets from pans. Lightly sift cocoa powder over tartlets and serve.
