Two Cups Flour

Fresh Lemon Curd
2 Make the lemon curd several hours or even the day before you plan to assemble the cake. The curd will need time in the fridge to cool and set up.
Lemon and Elderflower Cake
8 Preheat oven to 350℉ (180℃) | 160℃ Fan Convection. Prepare three 6-inch cake pans with a light coating of butter and flour, line the bottoms with parchment paper.
3 Place the lemon zest and granulated sugar in the bowl of a stand mixer. Work the zest into the sugar with your fingertips to help the sugar absorb the lemon oil. 
4 Add in the flour, baking soda and salt. Mix together on medium-low using the paddle attachment. 
5 Mix the softened butter one tablespoon at a time, mixing on medium speed until the mixture resembles coarse wet sand or fine breadcrumbs.
6 In a large measuring cup or bowl with a spout, stir together the oil, milk, eggs, egg whites, vanilla, and elderflower syrup. Use a fork or small whisk to make sure everything is evenly dispersed. 
7 Slowly pour the wet mixture into the sandy flour mixture in 2-3 additions, beating on medium speed just until smooth. The batter should be lighter in color and pourable.
9 Divide the batter evenly between the three pans. Weigh the batter to be exact for even cake layers.
10 Bake the cake layers on the middle oven rack for 30-32 minutes. Cakes are baked when the sides pull away from the pan, a toothpick inserted into the center comes out clean and the cake springs back when gently pressed with finger tip. *Check cakes at the 30-minute mark as all ovens bake differently.
11 Place cake pans on wire rack to cool for 15 minutes before removing and placing cake layers directly onto wire rack to cool completely.
17 Once cooled, cakes can be tightly wrapped in plastic wrap and refrigerated until ready to assemble. Cold cake layers are also easier to level with a knife. 
Vanilla Swiss Meringue Buttercream Frosting
13 While cake layers cool, make the buttercream frosting. 
15 Mix 1 tablespoon of elderflower syrup into the buttercream, along with the vanilla bean paste.
Assemble Lemon & Elderflower Cake
16 Use a serrated knife to trim the tops of each cake layer, so they are level. Remove the parchment paper form the bottoms. Place a single cake layer right side up on a serving plate.
19 Fill a large piping bag with the buttercream frosting and snip off the end. 
18 Spread a thin layer of frosting across the top with an offset spatula. Pipe a border of frosting around the edges, then fill the center with about 1/3 cup of lemon curd. Careful to keep the curd inside the frosting border. 
20 Gently place the second layer of cake, right side up onto the lemon curd. Repeat with a thin layer of frosting, pipe a border of frosting, and then fill the center with lemon curd. 
21 Invert the top piece of cake upside down for a flat top and place it directly onto the lemon curd. 
22 Pipe a thin coating of frosting around the outside and top of the cake. Smooth with an offset spatula and chill the cake in the fridge for 10 minutes. This is called a "crumb coat" and prevents cake crumbs from getting into the outer layer of frosting. *optional step, but makes decorating easier. 
23 Cover the top and sides of the cake with another layer of frosting. 
24 Place any additional frosting in a piping bag with desired piping tip and decorate.