Step 1

Preheat oven to 350°F.

Step 2

Grease three 9-inch cake pans and line the bottoms with circles of parchment paper.

Step 3

Melt chocolate in a small saucepan over low heat, or melt in the microwave in a microwave-safe bowl at 30 second intervals. Set aside.

Step 4

Beat the butter with an electric mixer until creamy. Add sugar and beat until combined. Add eggs one at a time, beating each until just incorporated. Stir in melted chocolate and vanilla.

Step 5

Sift together the flour, cocoa powder, baking soda and sea salt; add to the butter mixture alternately with the buttermilk, beginning and ending with the flour mixture. With the mixer running, gradually add in the hot water. Mix until the batter in consistent (batter will be thin).

Step 6

Pour batter into prepared pans and bake for 30 minutes, or until a toothpick tester comes out clean. Cool in the pans on wire racks 10 minutes; remove cakes from the pans and cool completely on wire racks.

Step 7

In a medium saucepan, whisk together flour and 1/2 cup of the milk until smooth. Set over medium heat and let cook for 3 minutes, or until slightly hot. Whisk in remaining milk and salt. Whisk constantly until the mixture is thickened (almost like pudding). Remove from heat and pour into a separate bowl. Place in the refrigerator until completely chilled. When cool, mixture will be thick like set custard.

Step 8

In a large bowl (or bowl of a stand mixer, if you have one) beat the butter and sugar together until fluffy. Beat in chilled flour mixture one tablespoon at a time on medium-high speed. When all the flour mixture is added beat on high speed until light and fluffy, about 6 minutes. Rub a little frosting between your fingers, if sugar granules remain, beat for 4 more minutes, or until granules cannot be detected with fingers. Beat in the pistachio paste a little at a time until well incorporated.

Step 9

Place a chocolate cake layer on a cake stand or serving plate and top with three cups of pistachio filling. Top with another cake layer and spread the remaining 3 cups of filling over the second layer. Top with the third and final layer. Refrigerate the filled cake while you make the frosting.

Step 10

Combine the chocolate and whipping cream in a saucepan. Cook over medium heat, whisking occasionally. When the mixture is steaming and the chocolate is melted remove from the heat. Add the truffle oil (add more to taste, but be careful – it’s strong stuff!). Whisk the mixture until the chocolate is completely melted and incorporated into the cream. Pour into a shallow dish and chill until the mixture reaches a pudding-like consistency, about 1 hour.

Step 11

Beat the mixture with an electric mixer until it is thick and spreadable. Transfer 2/3 cup of frosting to a piping bag fitted with a large star tip. Frost the cake evenly with the remaining icing and pipe rosettes around the top edge of the cake. Top every other piped rosette with a black truffle bonbon. Sprinkle the top with ground pistachios. Place the cake on a large baking sheet and toss ground pistachios along the bottom edge of the cake, allowing the excess to fall back into the pan.

Step 12

Store the cake in the refrigerator. Serve chilled or let slices come to room temperature before serving – it’s good both ways!