Step 1

Crust: Preheat oven to 350°F. Pulse wafers in a food processor until finely ground. Add butter, sugar, and fleur de sel; pulse until evenly moistened. Press evenly into the bottom and up sides of a 9-inch fluted tart pan with a removable bottom. Bake until firm, 8 to 10 minutes. Transfer to a wire rack; let cool completely.

Step 2

Candied orange: With a vegetable peeler, remove orange zest in strips (leaving white pith behind); finely slice. In a small saucepan, bring 1/2 cup sugar and 1/2 cup water to a boil, stirring until sugar has dissolved. Add zest; reduce heat to medium and simmer until tender, about 10 minutes. Remove from heat and strain, reserving syrup (you should have about 1/2 cup).

Step 3

Transfer zest to a large plate and spread in an even layer; let cool completely. Toss with remaining 2 tablespoons sugar. Zest can be stored up to 1 week in an airtight container at room temperature.

Step 4

Caramel and ganache: Combine cream and orange zest in a saucepan. Bring to a simmer over medium-high heat, then remove from heat, cover, and let steep 1 hour. Strain. Measure 6 tablespoons; refrigerate the rest.

Step 5

Bring reserved orange syrup, sugar, 2 tablespoons water, salt, and cream of tartar to a boil in a small saucepan. Cook, gently swirling pan occasionally, and washing down sides with a wet pastry brush (to prevent crystals from forming), until mixture turns deep amber, 9 to 11 minutes. Remove from heat and slowly add reserved 6 tablespoons infused cream (mixture will bubble and steam). Stir in butter until smooth. Pour caramel into cooled crust; let cool 15 minutes. Refrigerate until firm, at least 4 hours or, covered, up to 2 days.

Step 6

Place chocolate in a medium heatproof bowl. Bring 1/3 cup reserved infused cream (saving any remainder for another use) to a simmer in a small saucepan over medium-high heat. Pour over chocolate and let stand1 minute, then stir until smooth. Remove tart from refrigerator and spread ganache over caramel to edges with an offset spatula. Return to refrigerator until chocolate is set, about 30 minutes.

Step 7

Remove tart from refrigerator and let stand at room temperature 15 to 30 minutes. Chop 2 tablespoons candied zest and sprinkle over chocolate. Sprinkle with fleur de sel and more chopped candied zest; remove from pan, slice, and serve.