Fill the bottom of a double boiler with 1 to 2 inches of water. You can also make your own double boiler using a saucepan and a bowl that fits just inside the pan. Make sure that the bowl does not touch the water in the pot below. Place the pan on high heat and bring to a boil.
Reduce to a simmer over medium-low heat.
Place the top of the double boiler on top. Whisk together egg yolks, lemon zest, lemon juice, salt, and lavender extract. Continue to whisk as the curd cooks. By whisking constantly, you’ll prevent the egg yolks from curdling. Cook until the mixture thickens like a thicker cream sauce. It should easily coat the back of a wooden spoon at this point. It will take about 10 minutes.
Remove from heat and whisk in the butter until melted.
Optional step: pour the curd through a fine mesh sieve for the creamiest lemon curd!
Pour into a bowl and cover with plastic wrap, pressing the plastic wrap to adhere to the top of the curd. This will prevent a film from forming.
Let cool completely. The curd will thicken as it cools. Remove plastic wrap and store in an airtight container or jar.
make vanilla bean cake
Preheat oven to 350˚F. Grease and flour two 9-inch cake pans. Set aside.
In a large bowl, sift the flour, baking powder, baking soda, and salt together. Set aside.
In the bowl of an electric stand mixer, using the paddle attachment, beat butter on medium speed with the sugar until it forms a soft paste, approximately 1 minute. Add the eggs one at a time, beating after each until incorporated, making sure to scrape down the sides of the bowl as needed.
On low speed, alternate between adding the flour mixture and the buttermilk, working in two batches for each. Once fully incorporated, beat in the vanilla bean paste.
Divide the batter evenly between the two cake pans. Bake in the oven for 25 to 30 minutes or until a cake tester comes out clean. Transfer the pans to wire racks and cool for at least 15 to 20 minutes, before turning out the cakes to cool completely.
Once the cakes are cool, use a serrated knife to cut off the domes of each cake so the cake tops are flat. Set aside.
make lavender cream cheese frosting
Beat together cream cheese and butter in the bowl of a stand mixer fitted with the paddle attachment, until smooth. This will take about 2 to 3 minutes.
Add powdered sugar, vanilla extract, and champagne. Beat on a low speed for about 30 seconds, then increase speed to high. Beat together until smooth and fluffy.
assemble
Place the bottom layer of cake on a platter or cake stand. Spread the frosting in a thin layer in a ring on top, leaving the center empty. Fill the center with lemon curd, spreading in an even layer.
Top with the other cake and frost all over with remaining frosting. Decorate as desired with sliced lemons. Serve.
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notes
Store cake in an airtight container for up to 3 days at room temperature, up to 5 days in the refrigerator.