Preheat your oven to 180°C.
Melt the butter in a saucepan then continue simmering until you get brown butter. Set aside.
Place the flour, ground hazelnut, brown sugar, baking powder and fleur de sel in a large bowl. Mix.
Add the Greek yogurt, single cream, egg yolk and Grand Marnier. Mix thoroughly.
Add the brown butter and mix again.
Whip your egg whites and add a tablespoon of this whipped egg white mixture to your dough and mix well. This will loosen the dough.
Add the rest of the whipped egg whites and mix gently with a spatula.
Pour into your two “heart” moulds lined with baking paper.
Bake for about 25 minutes.
Take out of the mould after your cake finishes baking in the oven and leave to cool on a rack.
While baking, heat the 25 g of sugar along with the juice from one of the two oranges, and simmer to make a syrup.
Cut the second orange into thin slices and cook them for about fifteen minutes in the syrup.
Set the drained blood orange slices aside on a plate.
Continue to heat the syrup until it has a somewhat thick consistency and let it cool.
When the cake and syrup are cold, whip your whipped cream. As soon as the whipped cream begins to thicken, add the mascarpone and icing sugar.
Stop whipping when your whipped cream is firm.
Then garnish your nut cake with whipped cream and arrange the blood orange slices on top.
Serve with the syrup or store in the fridge until you want to eat it.