Beat the butter with a hand mixer on high speed until smooth. Gradually add the sugar, vanilla sugar and salt.
Stir until a bound mass is formed. Gradually stir in 2 eggs. Mix flour with cornstarch and baking powder, sift and stir into the batter in batches at medium speed. Stir in poppy seed cake mix (save 1 tsp. ). Place 2 paper cups inside each, divide the batter between the cups and place on the baking tray in the oven. Bake at top/bottom heat 180 degrees, hot air 160 degrees (preheated) for about 25 minutes.
Then let cool on a cake rack. For the glaze, mix the powdered sugar with water and the teaspoon of poppy seed mixture to a thick consistency. Brush the muffins with it and let the glaze set.