In a food processor or a bowl, whirl or mix flour, 1/4 cup sugar, cocoa, and 1/4 teaspoon salt until blended. Whirl in butter, or rub in with a pastry cutter or your fingers, until well blended. Add whole egg and whirl or mix just until dough starts to come together in a ball.
Scrape dough out onto a well-floured board and knead briefly to bring together. Divide into six equal pieces. With well-floured hands, press a piece of dough evenly over bottom and up sides of each of six 4-inch tartlet pans with removable rims. Place pans on a baking sheet and freeze until firm, at least 30 minutes (see notes).
Place baking sheet with tartlet pans directly into a 350° regular or convection oven (do not thaw). Bake until the crusts feel dry to the touch, 15 to 18 minutes. Cool in pans on a rack for 10 minutes, then remove pan rims and gently loosen crusts from pan bottoms with the tip of a sharp knife. Set crusts on rack to cool completely.
Meanwhile, in a 2- to 3-quart pan over medium-high heat, bring about 1 inch water to a boil. Place chopped chocolate in a bowl, remove pan from heat, and set bowl over water. Stir occasionally until chocolate is melted and smooth.
In a 3- to 4-quart pan over medium heat, whisk 6 tablespoons sugar into milk; bring just to a simmer, stirring often. In a small bowl, whisk cornstarch into egg yolks. Whisk about 1/2 cup hot milk mixture into yolk mixture, then whisk yolk mixture back into milk mixture. Stir constantly until simmering, then cook, stirring often, until very thick, about 1 minute. Stir in melted chocolate and vanilla. Scrape into a bowl and place a layer of plastic wrap directly on surface. Chill until cold, at least 1 hour (see notes).
In a bowl, with a mixer on high speed, whip crème fraîche, cream, and remaining 2 tablespoons sugar until soft peaks form. Peel bananas and cut into 1/4-inch-thick slices.
Spread a generous 1/4 cup chilled chocolate mixture in the bottom of each tartlet shell. Top with 6 to 8 banana slices (about a third of a banana per tartlet). Spread about 3 tablespoons more chocolate mixture over bananas in each tartlet to cover. Mound about 1/3 cup whipped cream mixture on each tartlet. Garnish with chocolate curls (see notes). Serve immediately or chill, uncovered, up to 4 hours.
Nutritional analysis per tartlet.
Crème fraîche adds a tangy edge to the whipped cream, but if it's unavailable, omit it and use 1 1/4 cups whipping cream. The crusts can be made through step 2 up to 1 week ahead; cover airtight and freeze, then uncover to bake. The chocolate mixture (steps 4 and 5) can be made up to 2 days ahead; cover airtight and chill. Use a vegetable peeler to make the thin chocolate curls for garnish, or sprinkle with unsweetened cocoa or chocolate sprinkles.