Add butter, some sugar, and salt to a bowl and start kneading. Add flour and keep kneading until a smooth dough forms. Wrap dough in plastic wrap and let rest in the fridge.
Add whole egg, egg yolk and sugar to a bowl and whisk. Add lemon juice, lemon zest and starch and whisk to combine.
Pre-heat oven to 170°C/340°F. Remove dough from fridge and roll out to a thickness of approx. 5-mm/0.2-in. Transfer dough to the bottom of a parchment-lined loaf pan and trim off any excess. Bake for approx. 15 min.
Remove loaf pan from the oven and spread filling on top of the baked dough. Bake a second time at 170°C/340°F for approx. 25 min. Remove from the oven and let cool completely. Slice to serve and dust with confectioner’s sugar if desired. Enjoy!