For the esterházy slices, beat egg whites with sugar until very stiff. Mix the hazelnuts with the flour and add to the egg white mixture.
Spread 1/2 cm thick on prepared baking sheet and bake in preheated oven at about 165 to 175 °C for 12 to 14 minutes until light. When cooled, cut into 4 strips and assemble each strip with buttercream. Brush top cover with fondant icing and decorate with chocolate.
For the buttercream, boil the milk with sugar. Reserve a little milk and mix it with the custard powder. Stir this mixture into the boiling milk and cook to pudding.
Pour hot pudding into a mixing bowl and cool slightly while stirring with a whisk. Add the chopped butter pieces or coconut oil to the still not quite cold pudding mixture and whip until fluffy. Then add the desired flavor. Keep the esterházy slices in a cool place.
I use this glaze very gladly, because it allows manifold nuances in terms of taste and has a beautiful shine.But be careful: Fondant glaze must not be cooked or heated too much! Suitable for all cookies and slices that you want to decorate or glaze.