Preheat oven to 350F. Grease and line the bottom three 6-inch cake pans with parchment paper. Set aside.

In the bowl of a stand mixer, combine granulated sugar and lemon zest. Rub the sugar and zest mixture between your fingers, working the sugar into the zest. This will release the essential oils into the sugar.

Add butter to the same bowl, and using the paddle attachment, beat the butter with the sugar at medium speed until fluffy, about 1 minute.

At medium speed, add eggs one at a time, only adding the next egg after the first one has been incorporated.

In a separate bowl, whisk together the dry ingredients.

In a measuring cup, combine sour cream, lemon juice, and vanilla.

With the mixer running on low, add the dry ingredients and wet ingredients in two increments. Mix until just combined.

Divide the batter evenly into the three prepared cake pans. Bake for 30 - 35 minutes, until a skewer inserted into the middle of each comes out clean.

Remove cakes from oven and let them cool on a cooling rack for 15 minutes. Run an offset spatula or butter knife around each pan and release cakes. Let cakes to cool completely before assembling.

Bring a pot of water to a light boil.

Whisk together egg whites and sugar in a large metal bowl.

Place bowl over the pot of boiling water, making sure the water doesn't touch the bottom of the bowl. Continue whisking until all the sugar has dissolved, about 2 minutes. The mixture should be hot to the touch.

Transfer egg white mixture into the bowl of a stand mixer fitted with a whisk attachment. Whisk on high speed for 8 - 10 minutes, until mixture becomes glossy and the meringue holds a medium - stiff peak.

Switch the whisk attachment for a paddle attachment. With the mixer on low, slowly add butter, a tablespoon at a time.

Once all the butter has been incorporated, add vanilla extract. Turn the mixture to medium - high speed and beat buttercream until fluffy and smooth, about 2 - 3 minutes.

Divide buttercream into small bowls and colour with gel food colouring as desired.

In a small pot, combine honey and water.

Bring to a light simmer, whisk over very low heat, stirring until combined.

Remove from heat and allow syrup to cool before using.

Level cake layers with a serrated knife.

Place a cake layer on cake stand or plate. Using a pastry brush, brush on a thin layer of honey simple syrup. Pipe a ring of buttercream around the edge of the cake layer then fill with lemon curd. Top with next cake layer and repeat.

Top with final cake layer and frost and decorate cake as desired.