Make the Thick Lemon Curd Filling

• For best results, make the lemon curd a day before using (or while baking the cake). Whisk the sugar and instant clearjel well together. Set aside.
  2 Tablespoons instant clearjel, 100 grams (½ cup) sugar
• Whisk the lemon juice and egg yolks together in a saucepan. Cook over medium-low heat, whisking frequently, just until it comes to a boil.
  ½ cup Fresh lemon juice, 4 egg yolks
• Remove from heat and add the 2 ounces room temperature butter to the lemon mixture, stirring until the butter is melted and well blended. Pour the hot lemon curd base through a fine mesh strainer to remove any lumps or egg strands.
  ½ stick (2 ounces) salted butter
• Slowly whisk in the instant clearjel mixture to the lemon curd, a little at a time, until the instant clearjel mixture is fully incorporated and dissolved. Stir in the lemon zest. The lemon curd will thicken, but will continue to thicken as it cools. This lemon curd is thicker and has a more textured appearance than traditional lemon curd due to the starch that thickens it, but it has a smooth mouthfeel.
  Zest from 2 large lemons
• Allow the lemon curd to cool and get cold in the fridge until you're ready to use it, ideally for two hours or overnight. Stir before using.