Optional: make your own jam and allow this to cool and set in the fridge for a few hours before using it in this recipe. Combine flour, powdered sugar, and salt in a food processor and pulse to combine. You can also do this by hand in a large bowl!
Add cold butter and pulse in the food processor until it looks like sand. If doing this by hand, just break up the cold butter with your fingers and crumble until you get to the sand-like consistency.
Add egg and vanilla extract and pulse until the dough starts to come together. If working by hand, add to your bowl, mix to distribute, and gently press together until your dough forms a ball. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Preheat your oven to 350 degrees F (175 C). Line a large baking pan or sheet pan with parchment paper.
After refrigerating, roll out the dough into one large rectangle, trimming the edges if needed (you can always bake the scraps separately if you roll them out again!). If you want, you can press the dough down into a baking tin instead of rolling it. We used a 9x13inch pan! The dough should be about 1/4inch thick.
Prick your dough with a fork to prevent bubbling and uneven rise. Bake for 25 minutes, until lightly golden brown around the edges.
While the dough is in the oven, make your marzipan topping. Combine egg whites, powdered sugar, and grated marzipan in a bowl and mix together until fully combined.
Once the dough is golden brown, remove it from the oven and top with jam and then add dollops of the egg white/sugar/marzipan topping. Spread it as evenly as you can.
You can also swirl the two layers together to create a cool pattern!
Put this back in the oven for another 20 minutes or until the top is brown and crispy. The top will look shiny but be hard to the touch! Cut into slices and enjoy!