A boat-shaped coconut macaroon is filled with lime curd and garnished with kaffir-lime leaves.
Grate zest from lemons and limes; squeeze juice into separate bowls. In a stainless-steel bowl, whisk zests, 1/2 cup lemon juice, 2 tablespoons lime juice, 3 eggs, and 1/2 cup plus 2 tablespoons sugar. Separate 1 egg; add yolk to egg mixture. Place white in a stainless-steel bowl, cover, and refrigerate. Whisk egg-sugar mixture until slightly thick, 3 to 5 minutes.
Place bowl over, not in, simmering water; cook, stirring constantly, until mixture starts to thicken, 8 to 12 minutes. Strain into a bowl. Stir in butter until melted. Place bowl in an ice-water bath. Let stand, stirring frequently, until cold, 30 to 45 minutes. This is the lime curd; refrigerate it while you make the tart shells.
Meanwhile, fold a piece of 10-by-12 inch aluminum foil in quarters. Fold long sides in by 1 inch, press flat, then fold in by 3/4 inch and leave standing for sides. Pinch ends closed; fold by 1/4 inch to seal, making canoe-shaped mold 5 inches long and 2 inches wide. Repeat to make 12 molds.
Heat oven to 325 degrees. Spread coconut on a baking sheet; toast until lightly browned, stirring frequently, 15 to 20 minutes. Cool on a wire rack. Raise oven heat to 350 degrees.
Separate remaining 3 eggs. Add whites to the bowl with reserved white (save yolks for another use). Add remaining cup sugar, the honey, and vanilla, and whisk together. Place bowl over, not in, simmering water. Cook, whisking constantly, until warm to the touch and sugar dissolves, 2 to 4 minutes.
In an electric mixer, beat egg-white mixture on high speed until stiff peaks form, 6 to 8 minutes. Using a rubber spatula, fold in toasted coconut, flour, and minced leaves. Use a pastry bag to pipe mixture into foil molds, filling them flush with the edges. With a dampened finger, press a well in the center about 4 inches long.
Place filled molds on a baking sheet. Bake until golden brown, 15 to 20 minutes. Let stand on a wire rack until very cool, about 30 minutes, before removing foil; cool shells completely before filling with lime curd.
Before filling shells, press the center back down. Fill each shell with about 1 tablespoon lime curd. Serve immediately or refrigerate up to 1 hour before serving.
Both the tarts and lime curd may be made up to 2 days ahead and refrigerated. Assemble just before serving.