Preheat the oven to 180 °C top and bottom heat. Grease 2/3 of the glasses with butter and sprinkle with flour, knock off excess flour. Place the rubber rings in cold water.
Peel and quarter the apples, remove the core and chop them into small pieces. Sprinkle with 2 tablespoons of lemon juice and place the grated peel on top. Put the remaining ingredients, except the apples, in a mixing bowl.
Mix everything with a hand mixer with whisk for a short time at the lowest level, then at the highest level for about 2 minutes to a smooth dough. Fold in apple pieces with lemon juice and lemon grated. Spread the dough evenly on the glasses, filling each glass only half full.
Clean the edges of the glass. Bake the glasses on the rack on the middle shelf of the oven for about 30-35 minutes.
Remove the glasses from the oven and close them immediately. To do this, place a wet rubber ring on the inside of each glass lid and fix the lid to the glass with 2-3 clamps. Let the mini cakes cool down in the closed jars on a cake rack.
The cake keeps for about 2 months in the glass.