Prepare the Cake Layers:

1. Preheat oven:
   • Set the oven to 350°F (175°C). Grease and line a 9-inch round cake pan with parchment paper.
2. Make the batter:
   • In a large bowl, beat 3 eggs with 3/4 cup sugar until light and fluffy. Gently fold in the melted butter, flour, baking powder, and salt until just combined.
3. Bake:
   • Pour the batter into the prepared pan and bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean. Cool completely, then cut into circles to fit your serving molds or ramekins.

Make the Meyer Lemon Mousse:

4. Prepare the gelatin:
   • In a small bowl, sprinkle 3 tsp gelatin over 4 tbsp water. Let bloom for 5 minutes.
5. Cook the lemon mixture:
   • In a saucepan, whisk together Meyer lemon juice, lemon zest, egg yolks, and 3/4 cup sugar. Cook over medium heat, stirring constantly, until the mixture thickens. Remove from heat and stir in the bloomed gelatin until dissolved.
6. Fold in whipped cream:
   • Allow the lemon mixture to cool slightly, then gently fold in the whipped heavy cream until fully combined.

Assemble the Mousse Cakes:

7. Layer the cake and mousse:
   • Place a pistachio layer (or cake circle) in the bottom of each serving mold or ramekin. Add a layer of Meyer lemon mousse on top. Repeat layers if desired.
8. Chill:
   • Refrigerate the assembled mousse cakes for at least 6 hours, or overnight, until set.

Serve:

9. Garnish and enjoy:
   • Remove the mousse cakes from molds and garnish with chopped pistachios, lemon slices, or zest. Serve chilled for a luxurious dessert experience.