In a large bowl, whisk by hand until well combined eggs, egg yolks, flour, sugar, lemon zest and salt. Set aside.

In a small sauce pan, place heavy cream on medium heat until it starts to boil, remove from heat and mix in vanilla extract.

Slowly pour 1/3 of the heavy cream into the egg mixture and whisk constantly. Make sure you DO NOT pour all the heavy cream at once, since the high temperature will make the eggs cook.

Slowly pour the 1/2 of the remaining heavy cream, whisking until combined. Repeat with the remaining boiled heavy cream.

Cover the bowl with a plastic wrap and let it cool on the counter for 20 minutes, before transferring to the fridge for one hour.

Preheat oven to 350F.

In a food processor or blender grind the biscoff cookies until you get 3 cups of crumbs.

Mix the crumbs with melted butter and granulated sugar with a rubber spatula in a medium bowl until combined.

Press the mixture into the bottom of a 10inch spring form cake pan and slightly up the sides. Make sure it is tight and compact, otherwise the custard will leak.

Pre-bake the crust for 7 minutes at 350°F (177°C), before adding the custard filling.

Note: For a no-bake dessert, chill the crust for 2 hours before using in your recipe.

Butter the edges of the spring form pan. Layer the blueberries on the biscoff crust.

Remove the custard from the fridge and pour it into the pan. Bake for 50 minutes, until golden brown.

Remove cake from the oven and place on a wire rack to cool completely.

The sauce can be made while the cake is cooking or many days in advance.

In a large saucepan over medium heat, combine blueberries, water and sugar. Stir frequently, but careful not to crush the berries, bring to a low boil.

In a small bowl, mix the cornstarch with cold water until combined.

Slowly stir the cornstarch into the blueberries, careful not to crush them. Simmer until the homemade blueberry sauce is thick enough to coat the back of a metal spoon, about 10 minutes.

Remove from heat and gently stir in vanilla.

If making the buttercream frosting, let the sauce cool completely before topping the cake.

Note: this makes a lot of sauce, save the rest for other recipes, or cut it in half.

Mascarpone Lemon Buttercream:

Place softened butter and mascarpone into the bowl of an electric stand mixer that has been fitted with the wire attachment. Turn the mixer on the medium setting and cream until it smooth and combined, 2 - 3 minutes.

Add sugar, ½ a cup at a time. Add lemon zest and a pinch of salt and whisk until well-incorporated.

If the frosting is too thick add heavy cream one tablespoon at a time until it has reached the desired consistency.

Top the cooled cake with the lemon mascarpone buttercream, and pour the cooled blueberry sauce on top of the frosting.