Preheat the oven to 350 F.
For the cake: Beat the butter. Add the remaining cake ingredients and beat until smooth. Divide the mixture between two 7-inch round cake pans and level the surface.
Bake the cakes on the center oven rack 30 to 35 minutes, or until firm in the center.
For the syrup: Pour 2 tablespoons water into a small saucepan; add the sugar. Place the pan over low heat and stir until the sugar dissolves. Increase heat and bring mixture to a boil; boil rapidly for 1 minute, then remove pan from the heat and add the lemon juice.
When the cakes are baked, remove from oven and pour the syrup over them. Leave the cakes in the pan for about 10 minutes so the syrup can soak in. Transfer to a wire rack to cool completely.
Cut each cake in half horizontally to create 4 thin layers. Spread the lemon curd on the layers and sandwich together.
For the frosting: Beat the butter until very soft. Add the remaining frosting ingredients and beat well until fluffy. Spread the frosting over the top and sides of the cake.