Preheat oven to 350F. Grease and flour three 6" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and lemon zest until smooth. Add sugar and beat on med-high until pale and fluffy (approx 3mins).
Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture with milk & lemon juice, beginning and ending with flour (3 additions of flour and 2 of milk & lemon juice). Fully incorporating after each addition.
Spread batter evenly between the prepared pans and smooth the tops.
Bake for about 30-35mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Place eggs and sugar into a small pot, whisk to combine. Add lemon juice, zest, and butter. Cook over medium-low heat whisking constantly until mixture thickens and coats the back of a spoon.
Transfer to a glass bowl and lay plastic wrap directly on surface to prevent a skin from forming. Chill for 2 hours to set.
Vanilla Ermine Frosting
Place sugar, flour, and salt into a medium saucepan. Whisk to combine.
Add milk and vanilla, stir to combine. Cook over medium heat, stirring constantly, until the mixture boils. Continue cooking while stirring constantly for 1-2 more minutes until the mixture thickens to a pudding-like consistency. Remove from heat.
Pour into a bowl and place plastic wrap directly on top of to prevent a skin from forming.
Cool to room temperature.*
Using a stand mixer or hand mixer, beat the (room temperature) butter on high until pale and fluffy (3 mins).
Add the (room temperature) flour mixture 1 Tbsp at a time, incorporating well after each addition. Beat for 2-3mins until smooth and fluffy.
Place one layer of cake on a cake stand or serving plate. Poke holes into the cake using a bamboo skewer and drizzle with simple syrup if desired.
Spread a thin layer of buttercream on top of the layer and pipe a border around the outside to hold the lemon curd in. Fill with about 1/3 of lemon curd. Repeat with next layer.
Place final layer on top and do a thin crumb coat on the cake. Chill for 20mins.
Frost the top and sides of the cake. Smooth with an icing scraper.
Pipe rosette dollops using a 1M tip with remainder of frosting. Chill for 20mins. Fill the top with the remaining lemon curd.
Decorate the bottom with thin sliced of lemon and sprinkle the top with lemon zest if desired.
* The flour mixture can be made a day in advance. Place it in the fridge overnight and bring to room temperature before using.