Step 1

Preheat oven to 350 degrees F. Butter and flour three 6 inch round pans. Whisk together the flour, baking powder, baking soda, and salt. Set aside.

Step 2

Mix fresh lemon juice and milk in a small bowl and set aside.

Step 3

Pulse sugar and zest of a lemon in a food processor until zest is all minced in.

Step 4

Cream butter and sugar in a stand mixer using a paddle attachment.

Step 5

Add vanilla and eggs one at a time mixing until incorporated before adding the next. Scrape the bowl down.

Step 6

Add flour and milk mixtures to the butter in alternating batches.

Step 7

Use a spatula to fold powdered sugar and lemon zest into the mascarpone. Mix until fully incorporated.

Step 8

Whip the cream and a drop of vanilla until thickened and mix into the mascarpone mixture then set aside.

Step 9

Whist together egg whites and sugar in a glass bowl. Set over a pot of simmering water making sure the water doesn't touch the bowl.

Step 10

Whisk over hear until a thermometer reads 165F. Transfer mixture to a stand mixer fitted with a whisk attachment and mix on high for a few minutes then stitch to medium low until room temp.

Step 11

Stitch to a paddle attachment and add butter a tablespoon at a time. Mix in lemon zest and frosting is ready. Always give meringue-based frostings a quick whip just before using.

Step 12

If you're doing an ombré decoration scheme then separate the buttercream into 4 batches. One will be uncolored and the other three various intensities of yellow.

Step 13

Add mascarpone filling onto first layer then place second layer on and repeat.

Step 14

Pipe the most intense yellow onto the bottom then the medium and light yellows followed by the uncolored buttercream on top. Smooth and pipe yellow dollops on top using a star tip.