Defrost the cherries. Coat glasses with oil. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Cream butter, sugar and lemon juice with the whisks of the mixer.
Stir in the eggs one by one. Stir in flour, baking powder and starch.
Chop the chocolate and fold into the dough with the cherries. Fill the glasses in the same way. Bake in the oven for about 35 minutes. Cover and preserving rings simmer in boiling water for 2 minutes. Drain.
Remove the cake. Close jars immediately with preserving rings, lids and clips. Let them cool down.