As with meringues, creating a foam for a souffle requires a very clean bowl and whisk, with not a speck of fat or yolk.
Place a rack in the bottom third of oven and preheat to 425°.
Butter the inside of an 8-inch souffle pan, then dust with granulated sugar. Also butter and sugar a 3-inch-high strip of aluminum foil, then wrap it to create a collar around the dish; crimp to secure.
Melt chocolate in a double boiler set over simmering water; remove from heat. In a separate saucepan, whisk half-and-half and flour together; place over high heat and cook, whisking constantly, until the mixture begins to thicken, about 3 minutes. Remove from heat. Cool 2 minutes and whisk in 3 tablespoons butter, coconut extract, shredded coconut, egg yolks, and melted chocolate.
Wet a paper towel with white vinegar and wipe the bowl and whisk of a mixer. Add egg whites and salt to the bowl and beat on medium-high speed until soft peaks form, about 1 minute. Very slowly stream in ½ cup granulated sugar, whisking constantly, until firm and glossy peaks form, about 1 or 2 minutes longer.
Pour about 1 cup of the chocolate mixture into the bowl, aiming for the side, and gently fold it into the egg whites. Then, working in three batches, repeat the process with the remaining chocolate mixture. Mix thoroughly, but gently; you should still have light streaks in your mixture when you’re done.
Pour into souffle pan and put into oven, immediately lowering heat to 375°. Bake 50 to 60 minutes, or until a bamboo skewer inserted into the middle comes out clean. Cool a few minutes before carefully removing foil collar. Dust with confectioners’ sugar and serve at once.