Grease a 25cm savarin tin with butter and dust it with flour.
Sift flour into a bowl, add yeast, and mix with room temperature milk.
Beat eggs and sugar until fluffy, then stir in zest, vanilla, salt, and melted butter.
Combine the egg mixture with the flour mixture and whisk until smooth.
Cover the batter and let it rise for 20 minutes.
Knock back the batter with a spoon and pour into the prepared tin.
Cover with oiled cling film and let rise in a warm room for 90 minutes.
Preheat the oven to 200°C.
Bake the savarin for 40 minutes before turning out onto a tray.
Simmer orange juice, liqueur, 250ml water, and sugar in a pan for 5 minutes.
Slowly pour the syrup over the cake and transfer it to a plate.
Whip cream with sugar and cinnamon until peaks form.
Pipe the cream into the center of the savarin using a star nozzle.
Decorate with thin orange slices and chopped pistachios.