Pistachio Truffle Layer Cake with Pistachio Swiss Meringue Buttercream

Pistachio truffle layer cake - pistachio-infused white cake layers filled with a whipped chocolate truffle ganache and frosting with pistachio Swiss meringue buttercream.

Whipped Ganache:
5 oz semi-sweet chocolate, chopped (140 g)
¾ cup heavy cream (180 mL)
1 pinch sea salt

Pistachio Cake:
2¼ cups + 2 Tbsp. cake flour (310 g)
2½ tsp. baking powder
¼ tsp. sea salt
¾ cup unsalted butter, room temperature (170 g)
¾ cup pistachio paste, stirred (180 mL)
1¼ cups sugar (250 g)
1¼ tsp. vanilla bean paste
3 large eggs
½ cup + 2 Tbsp. whole milk (150 mL)

Pistachio Swiss Meringue Buttercream:
6 large egg whites
2 cups sugar
4 tsp. pure vanilla extract (400 g)
3 cups unsalted butter, room temperature (675 g)
⅔ cup Pistachio Paste, stirred (160 mL)

Assembly:
pistachios, chopped (optional)

For the Whipped Ganache:
Place the chocolate in a heat-safe bowl and set aside.

In a small saucepan on the stovetop, bring the heavy cream to a low simmer. Remove from the heat and pour over the chocolate. Allow to sit for 30 seconds, then whisk until smooth. Add the salt and stir to combine. Allow to cool for 2 hours.

Preheat oven to 350°F (180°C/160°C fan, Gas Mark 4). Grease three 6" cake pans and line with parchment paper. Set aside.

For the Pistachio Cake:
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

Combine the butter, pistachio paste, and sugar in the bowl of a stand mixer fitted with the paddle attachment on medium-high speed for 3-5 minutes, or until light and fluffy. Scrape down the sides of the bowl.

Add the vanilla and mix on medium-low speed until combined. Add in the eggs, one at a time, mixing well after each addition. Scrape down the sides of the bowl.

Add the flour in three additions, alternating with the milk. After the last flour addition has been added, mix on medium for no more than 30 seconds.

Evenly divide the batter among the prepared cake pans. Bake for 25-28 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool in the pans on a wire rack for 10-15 minutes, then remove from the pans and allow to cool completely. Level the tops of the cakes with a serrated knife once cooled.

For the Pistachio Swiss Meringue Buttercream:
Combine the egg whites and sugar in the bowl of a stand mixer and whisk to combine. Place the stand mixer bowl over a double boiler, making sure the bottom of the bowl does not touch the boiling water in the pan below. Heat the egg mixture until it reaches 160°F (70°C) on a candy thermometer, whisking occasionally.

Place the stand mixer bowl back on to the stand mixer mix with the whisk attachment for 8-10 minutes on high speed, or until the outside of the bowl has cooled to room temperature.

Decrease the mixer speed to low and add in the vanilla. Add in the butter a few Tbsp. at a time, making sure each addition is fully incorporated before adding the next. Once all the butter has been added, switch the whisk attachment to the paddle attachment. Mix on medium-high speed until smooth and fluffy, about 3-5 minutes. Add in the pistachio paste and mix until combined.

Assembly:
Once the ganache has cooled, scrape into the bowl of a stand mixer and mix with the whisk attachment on medium-high speed for 30-60 seconds. Stop the mixer and mix by hand with a balloon whisk until it is lighter in texture and thick enough to clump onto the whisk. Use immediately.

Place one cake layer on a cake board or serving plate. Fill a piping bag fitted with a plain round tip with the pistachio buttercream. Pipe a ring around the top of the cake to create a dam. Fill the ring with ¾ cup of the whipped chocolate ganache and smooth it into an even layer with an offset spatula. Top with the second cake layer and repeat. Then, place the final cake layer on top.

Frost the tops and sides of the cake with a thin layer of pistachio buttercream for the crumb coat. Place the cake in the refrigerator and chill for 15 minutes.

Decorate the Cake:
Frost the cake with a thin layer of buttercream. To decorate the sides with a wavy ruffle technique, fill a piping bag fitted with a petal tip with the buttercream. With the opening of the tip facing down and the narrow side of the tip facing away from the cake, start at the bottom of the cake and pipe waves up the side of the cake. Pipe waves around the whole cake, trying to follow the curves of the last wave as you do. Smooth the top of the cake with an offset spatula. Sprinkle the top of the cake with pistachios, if desired.