Set the oven temperature to 150°C.
Stir together the butter, sugar and vanilla sugar. Add the oil, a little at a time.
Mix the ammonium carbonate with a little of the flour and stir into the rest of the mixture. Add the remainder of the flour and work to a smooth dough.
Add the lemon zest and the poppy seeds. Mix well.
Divide the dough into two parts and roll into two strips. Cut into 50 pieces and roll into buns. Place the buns on a sheet of baking paper. Flatten them using your hands.
Bake at the centre of the oven for 20 minutes.
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