Bittersweet chocolate gets a boost of nuanced sweetness from honey, which gives these truffles a silken texture.
Line a 13-by-9-inch baking dish with plastic wrap, leaving a 3-inch overhang on all sides. Heat honey, cream, vanilla seeds and pods, and a pinch of salt in a medium saucepan over medium heat, stirring frequently to dissolve honey, until mixture just begins to simmer. Remove from heat, cover, and let steep for 20 minutes.
Place chocolate in a food processor. Uncover cream mixture, and bring to a simmer. Immediately strain through a fine sieve over chocolate; discard vanilla pods. Let chocolate mixture stand for 1 minute. Process until smooth, add butter, and process until butter is incorporated, about 1 minute. The mixture should have the consistency of pudding.
Pour into baking dish, press plastic wrap directly on surface of chocolate mixture, and refrigerate until very firm, about 2 hours. Uncover, use plastic wrap overhang to lift out hardened chocolate, and place on a cutting board. Dip a 1 1/2-inch round cutter in hot water, wipe dry, and cut out a chocolate round. Repeat, dipping and drying cutter frequently, with remaining chocolate mixture. If chocolate mixture becomes too soft, refrigerate again until firm. Rounds should be 1 inch tall; if necessary, trim tops. (Enjoy scraps, or melt again over low heat, chill in a lined baking dish, and cut.)
Roll each round in cocoa powder, shaking off excess, and transfer to a parchment-lined baking sheet. Cover, and refrigerate until ready to serve (up to 1 week). Just before serving, top with well-chilled buttercream flowers.