Make the Pistachio Cake:
1. Preheat Oven: Set your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or line with parchment paper.
2. Prepare Dry Ingredients: In a bowl, whisk together the finely ground pistachios, flour, baking powder, and salt.
3. Cream Butter and Sugar: In a large bowl, cream together the butter and sugar until light and fluffy.
4. Add Eggs and Vanilla: Beat in the eggs one at a time, then stir in the vanilla extract.
5. Combine Mixtures: Gradually add the flour mixture alternately with milk, beginning and ending with the flour mixture. Mix until just combined.
6. Bake: Pour the batter into the prepared cake pan and smooth the top with a spatula. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
7. Cool: Remove from the oven and let cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
Make the Meyer Lemon Mousse:
1. Bloom Gelatin: In a small bowl, sprinkle gelatin powder over cold water. Let it sit for 5 minutes to bloom.
2. Whip Cream: In a mixing bowl, whip the heavy cream and powdered sugar until soft peaks form.
3. Add Lemon Zest and Juice: Mix in the Meyer lemon zest and juice.
4. Incorporate Gelatin: Microwave the bloomed gelatin for 10 seconds until melted and clear. Gradually pour the melted gelatin into the whipped cream mixture while beating continuously until fully incorporated.
5. Color: Add yellow food coloring if desired, mixing until uniform in color.
6. Chill: Chill the Meyer lemon mousse in the refrigerator for about 1 hour, until it firms up slightly.
Assemble the Mousse Cakes:
