1. Sift together the flour and wheat starch. Set aside. In a small saucepan, combine the syrup ingredients (water and sugar) and bring to a boil. Cook until all the sugar has dissolved. Remove from the heat and let it cool to room temperature. Set aside.
2. Line the bottom and the sides of a 15 cm circle cake pan with parchment paper. Tip : Put a bit of butter on the inside of the cake pan, it helps the parchment paper stick to it. Set aside.
3. In a mixing bowl add eggs and sugar. Whisk to combine in a hot water bath with an electric mixer until the mixture whitens, doubles in volume and forms ribbons (about 5~6min). Remove from the hot water bath and slow down to low speed for a few seconds and then stop.
4. Gently add the sifted flour mixture from #1 and mix with a egg whisk until incorporated. (make sure the batter is free of lumps.)
5. Microwave the butter (15g) and milk (20ml) for 15 sec at 600W, mix until the butter is melted.. Add about 2 tbsp of cake mixture (from #4) and mix. Pour the butter mixture little by little into the rest of the cake mixture from #4. Mix gently with a spatula until it is homogeneous. Do not over mix.
6. Preheat the oven to 170°C. Pour the cake mixture into the prepared cake pan and gently tap the bottom of the pan on your counter top a few times to eliminate air pockets. Bake for about 20min or until done. Insert a skewer in the center of the cake : if there is no sticky butter residual, then your baking is done.
7. Remove from the oven and take the cake out of the pan. Let it cool to room temperature on the wire rack.
Meanwhile, make the lemon curd
8. In a medium bowl, whisk together egg and sugar. In an another microwave safe bowl, add lemon juice and microwave 20~25s at 600W. Gradually add the hot lemon juice into the egg mixture. Mix well after each addition. Add the corn starch (sifted) and mix.
9. Transfer the egg mixture into a cooking pot, using a sieve. Add the butter and cook over medium-low heat, stirring constantly, until the mixture has thickened. Transfer to a bowl and set aside.
And fix the whipping cream
10. In a mixing bowl, combine the whipping cream and sugar, beat at high speed until stiff.
NOTE : if your cream is less than 35% fat, I recommend you to add whipped cream stabilizer (I used "cremfix" here). In that case, mix the cream stabilizer to the sugar ahead of this step. Add when your cream is getting a soft peak, then continue to beat until firm-peak.
Put it all together
11. Using a sharp knife trim 2 to 3 mm from the top and the bottom of the cake to remove the brown surface layer. Then slice horizontally in 2 halves. Place 1 layer of cake, brush syrup from #1 on top, spread lemon cream, and then spread whipped cream. Repeat this one more time (cake+syrup+lemon). Finally add the top sponge cake layer, spread and whipped cream on top and sides of the cake.
12. Sprinkle with any nuts your like to decorate. (I used Pralin for this cake.) Let sit in the fridge until you serve.