Mix the butter with sugar, salt, vanilla sugar and the grated lemon zest with the whisk of the hand mixer or in the food processor until very frothy. Add the eggs one at a time, stirring in each egg for at least 2 minutes. Keep stirring until the sugar has completely dissolved. Mix the flour and baking powder, sieve and stir in briefly with the milk. Carefully fold in the drained mandarins.
Line a 12-cup muffin tin with paper baking cases or silicone cases. Pour the batter into the molds. Bake in the preheated oven at 175 ° C (convection 160 degrees) on the 3rd rack from the bottom for about 25 minutes, test with chopsticks. Cool on a wire rack.
For the topping, squeeze out the lemon juice, mix with the powdered sugar and decorate the muffins with the topping and pistachios.