First melt the butter on a low heat. Once it’s melted add the white chocolate and mix it till it’s one smooth mixture. Then crack the eggs in a bowl and also add the sugar and the salt. Now beat the eggs till it’s a thick and firm foam. You can also do this by hand, but you need some serious muscles and time for that… Then add the chocolate mixture to the beaten eggs and mix it for another 30 seconds. After that mix the flour with the baking soda on a sieve. Add it to the bowl. Then gently fold this together to keep it as airy as possible. Now pour it on a tray that’s covered with a silicon sheet and spread it. Bake it at 180 degrees Celsius for around 20 minutes till golden. Then let it cool down out of the fridge.