Preheat the oven to 350ºF

Grease the bottom and sides of the springform pan with some butter, then dust some of the almond flour (about 1 Tbs), which will stick to the buttered walls.

Melt the butter. Zest the orange, then juice it.

Separate the eggs. In a stand up mixer, beat the yolks with half of the sugar until frothy. Add in the melted butter, orange zest and juice and mix.

In a separate bowl, mix the almond flour, ground almonds and baking powder. Add the yolk mixture to the almond mixture and stir to combine.

In the bowl of the stand up mixer, beat the egg whites with the remaining sugar until stiff.

With a rubber spatula, gently fold the beaten egg whites into the yolks and almond mix. Pour this batter into the springform pan and bake for about 40 minutes (check at the 30 minute mark, because all ovens are different and therefore this baking time may vary).

Remove from the oven, let the cake cool down, then finish with a dusting of icing sugar and a few thin strips of orange peel, if desired.