How to Make It
Beat butter and sugar in the bowl of a stand mixer fitted with paddle attachment on medium speed until light and fluffy, 4 to 6 minutes. Add egg yolks in 3 batches, beating well after each addition and stopping to scrape sides of bowl as needed. Stir together flour, coconut, cocoa powder, baking powder, and salt in a medium bowl. Gradually add flour mixture to butter mixture, beating on medium-low until just combined. Divide dough into 2 equal disks, and wrap each disk tightly in plastic wrap. Chill at least 2 hours or up to 3 days.
Unwrap 1 dough disk, and place between 2 sheets of parchment paper. Roll to 1/4-inch thickness. Transfer rolled dough and parchment to freezer; freeze until hardened, about 30 minutes.
Preheat oven to 325°F. Place frozen dough and parchment on a work surface. Remove top sheet of parchment paper. Using a 2 1/2-inch round cookie cutter, cut 18 circles from dough, rerolling scraps as needed. Place dough circles 1 inch apart on 2 parchment-lined baking sheets. (Chill dough if it becomes too soft.)
Bake dough circles in preheated oven until firm to the touch, 10 to 14 minutes. Cool sablés on baking sheets 10 minutes. Transfer to a wire rack to cool completely, about 15 minutes. Repeat rolling, cutting, and baking with remaining dough disk to yield 36 sablés.
Bring 1 inch of water to a simmer in a large saucepan over medium-low. Combine chocolate and heavy cream in a medium-size heatproof bowl. Place bowl over simmering water, ensuring bottom of bowl does not touch water. Cook, stirring often, until mixture is smooth, 6 to 8 minutes. Remove saucepan and bowl from heat; remove bowl from saucepan, and let cool at room temperature 20 minutes. Transfer to fridge, and chill, uncovered, until hardened but still pliable, about 2 hours.
Using a 1 1/4-inch scoop (about 1 tablespoon), scoop ganache into semicircles, pressing mounds against side of bowl to flatten slightly. Place ganache mounds, flat sides down, on a parchment-lined baking sheet. (You should have 36 mounds.) Freeze until hardened, at least 1 hour or up to 1 week.
Make the vanilla marshmallow creme: Combine egg whites and 2/3 cup sugar in the bowl of a stand mixer fitted with whisk attachment. Beat on low speed to gently stir together egg whites and sugar (do not whip); continue beating on low while preparing sugar syrup in Step 8.
While egg whites are beating, stir together 1/4 cup water and remaining 1 1/3 cups sugar in a small saucepan over high. Cook, swirling pan occasionally (but not stirring), until a thermometer inserted in mixture registers 200°F, 4 to 6 minutes. When sugar syrup reaches 200°F, increase mixer speed to high, whipping egg whites to stiff peaks. (The goal is to have the sugar syrup heated to 240°F at the same time egg whites reach stiff peaks.) Once sugar syrup is at 240°F and egg whites are at stiff peaks, about 2 minutes, keep mixer on high speed and quickly stream sugar syrup into whipped egg whites. Beat on high 4 minutes. Reduce speed to medium-high, and beat 4 more minutes. Reduce speed to medium-low, and beat until mixture is 90°F, 15 to 20 minutes. (It’s important to gradually decrease speed of mixer. This allows mixture to cool slowly and achieve an elastic texture that is less brittle.) Add vanilla, and beat on low speed until combined.
Place cooled sablés on a baking sheet. Place 1 frozen ganache mound, flat side down, on center of each sablé. Transfer vanilla marshmallow creme, in batches, to a large piping bag fitted with a 1/2-inch round tip. Pipe a smooth mound of creme over ganache on 1 sablé, completely covering ganache and ensuring creme is sealed to sablé on all sides. Pipe 2 additional rounds of creme on top to achieve the traditional Krembo shape (about 1/3 cup creme for each Krembo). Repeat with remaining sablés and creme. Freeze piped Krembo until creme is set, at least 2 hours or up to 1 week.
Bring 1 inch of water to a simmer in a small saucepan over medium-low. Combine chocolate and coconut oil in a medium-size heatproof bowl. Place bowl over simmering water, ensuring bottom of bowl does not touch water. Cook, stirring often, until mixture is smooth, about 5 minutes. Pour chocolate mixture into a narrow deep vessel, such as a 2-cup liquid measuring cup. Cool at room temperature, stirring occasionally, until mixture reaches 80°F, about 1 hour. Working with 1 Krembo at a time, invert and dip creme into chocolate until chocolate is about 1/2 inch from sablé base. Quickly remove Krembo from chocolate, and hold inverted until excess has dripped off, about 15 seconds. Turn Krembo right-side-up; immediately sprinkle with about 1/4 teaspoon cacao nibs. Chill at least 30 minutes or up to 4 hours. Serve chilled.
Valrhona 66% fèves may be purchased at Whole Foods or online. Be sure to use a gluten-free flour blend that includes xanthan gum to ensure proper emulsification.