Jolts of cocoa and espresso flavor every layer: the filling, the frosting, the truffles are all infused with mocha.
Preheat oven to 350 degrees. Butter three 8-inch round cake pans (2 inches deep), and line with parchment cut to fit. Butter parchment, and dust with cocoa, tapping out excess. Sift cocoa, flour, sugar, baking powder, baking soda, and 1 1/2 teaspoons salt into the bowl of a mixer. Beat with mixer on low speed until just combined.
Raise speed to medium, and add eggs, buttermilk, water, oil, and vanilla. Beat until smooth, about 3 minutes.
Divide batter evenly among pans. Bake until cakes are set and a toothpick inserted into the centers comes out clean, 37 to 39 minutes. Let cool in pans set on wire racks for 15 minutes. Invert pans to remove cakes. Transfer cakes, right side up, to wire racks. Let cool completely.
Assemble the cake: Trim tops of cakes using a serrated knife to create even surfaces. Place 1 cake on a cake stand or a large round platter. Transfer half the mocha buttercream to a pastry bag fitted with a 1/8-inch plain round tip. Pipe buttercream around edge of cake to create a dam. Spread 1 cup mocha pastry cream over top of cake inside dam using a small offset or rubber spatula. Top with 1 cake, and repeat to cover top with buttercream and pastry cream. Top with remaining cake.
Spread a thin layer of buttercream (about 1 cup) on stacked cake to seal in crumbs, starting at the top and then frosting the sides, using a large offset spatula (this is called a crumb coat). Use spatula to smooth surface as much as possible. Refrigerate, uncovered, for at least 1 hour.
Transfer remaining buttercream to pastry bag. Apply another (thicker) layer of buttercream (about 2 cups), starting at the top and then frosting the sides using a large offset spatula. Smooth top with spatula, then smooth sides with a bench scraper. Refrigerate for 1 hour.
Mound truffles onto center of cake. Sprinkle sugar pearls onto cake around truffles.