Prepare the gelatin by placing the packet with 4 tablespoons of water in a small microwave-safe bowl. Let stand until thick, 3–5 minutes. Heat gelatin in the microwave for about 5 seconds. Keep heating in 5-second increments until the gelatin is liquid.

Meanwhile, in a stand mixer, whisk heavy cream, sugar, and vanilla on low until combined. Once gelatin is melted, increase speed on mixer to high, beating the cream mixture until soft peaks form. Add liquid gelatin to cream mixture in a constant, thin stream while mixer is beating. Continue whisking cream mixture until stiff peaks emerge, about 2–3 minutes. Add yogurt and sour cream and whisk again to combine. Cover and place mixture in fridge until ready to use.

Preheat the oven to 350°F. Line a half sheet pan with parchment paper.

Using a blender or food processor, grind the pistachios, sugar, and potato starch. Pulse to create fine crumbs, about 15–20 seconds, depending on the power of your machine; be careful not to turn it into a nut paste. Set aside.

Using a stand mixer with the whisk attachment, whisk egg whites until foamy. Slowly add confectioners' sugar to create stiff peaks.

Using a rubber spatula, gently fold nut mixture into egg mixture, adding little by little so as not to deflate the dacquoise. Transfer mixture onto prepared half sheet pan. Use an offset spatula to spread evenly.

Bake for 20 minutes or until golden brown; it is ready when a toothpick inserted in the center comes out clean. Let cool before cutting.

Invert dacquoise onto a cutting board and gently remove the parchment paper. Cut in half lengthwise. Cut each piece in half again lengthwise to make four long equal parts. Spread 1/3 of the cream filling onto one of the cut layers. Cover the filling with a second layer of dacquoise. Spread another 1/3 of the cream filling on top of the second layer. Cover the filling with a third layer of dacquoise. Spread the remaining 1/3 of the filling on top of the third layer. Place the last cut layer on top of the filling. Chill in the refrigerator.

Note: If preparing ahead of time, wrap in plastic wrap or parchment paper and freeze.

Cover gelatin with 1 tablespoon of water, allowing it to bloom. Set aside.

Meanwhile, whisk egg yolks with sugar until combined and light in color.

In a small saucepan, heat milk and bloomed gelatin until it just comes to a boil. The gelatin should be dissolved at this point. Pour half of the milk into egg and sugar mixture; whisk together. Pour milk, egg, and sugar mixture into the remaining milk in saucepan. Cook on low heat until mixture thickens and holds shape on the back of a spoon.

Meanwhile, beat heavy cream to stiff peaks.

Place chocolate in medium-sized mixing bowl. Using a strainer, pour milk mixture over chocolate, and mix until incorporated. Microwave 10–15 seconds at a time if white chocolate is still solid. Add extract or essential oil. (Lemon zest will do, as well. Add lemon zest to the cream the night before and then strain before using.) Allow to cool to room temperature.

Fold heavy cream into chocolate mixture.

Remove chilled cake, trim as desired, and frost with the chocolate mousse, covering entire cake evenly. Freeze and garnish with sliced, twisted lemons and crushed pistachios. Each component can be prepared ahead of time and assembled on the day of the family event or celebration. The chocolate mousse can be a stand-alone dessert, placed in small dessert cups or goblets garnished with nut brittles or pralines. I’ve made this with wild orange essential oil, served in a dessert glass layered with crushed hazelnut brittle with a candied orange on top.