Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper.
Melt the butter in a small saucepan, then set it aside to cool slightly.
Whisk eggs and sugar together in a large heatproof bowl. Place the bowl over a pan of hot water and use an electric mixer to whisk the mixture on high speed until it turns pale and creamy, leaving a trail when the whisk is lifted.
Remove the bowl from the heat and continue whisking until the mixture cools down.
Sift the dry ingredients: flour, baking powder, salt, and cornstarch into a separate bowl.
Fold in the dry ingredients: Gently fold half of the sifted dry ingredients into the egg mixture.
Add the cooled butter: Pour half of the cooled melted butter around the edges of the egg mixture and fold it in carefully. Repeat the process with the remaining dry ingredients and butter.
Pour the batter into the pan, smoothing the top to ensure an even surface.
Bake the cake for about 40 minutes, or until the cake has risen, feels firm to the touch, and starts to pull away from the sides of the pan. Let the cake cool in the pan for a few minutes before turning it out onto a wire rack to cool completely.
Slice the cake into three horizontal layers using a serrated or bread knife.
Assemble the cake: Reserve 3-4 tablespoons of whipped cream. Mix the remaining whipped cream with lemon curd and use it to sandwich the layers together. Spread the reserved whipped cream over the sides and top of the cake. Roll out fondant icing and cover the cake, trimming any excess. Decorate with crystallized flowers and ribbon.
