Grease the jar. Place 2 paper baking cups into each other in the recesses of a muffin tray (12 recesses). Preheat the oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2). Wash the orange, dry and finely grate the peel.
Cream 250 g butter, sugar, vanilla and orange peel with the whisks of the mixer. Stir in the eggs one by one. Mix flour, baking powder, 1 pinch of salt and poppy seeds. Alternately stir into the dough with the buttermilk.
Fill the preserving jar to almost 2/3 with dough. Spread the rest of the dough into the paper baking cups. Bake the cake in the jar in a hot oven for about 30 minutes (without lid), bake the muffins for 20-25 minutes.
When cake in a glass jar, check with a long wooden skewer after about 20 minutes to see if the dough is baked through. If there is still tough dough sticking to the skewer, bake the cake for another 5-10 minutes (not longer, otherwise the cake will get dry).
First remove the muffin tray, then the glass from the oven. Cake in the jar, let the muffins cool in the tins. Serve with apricot sauce.