Coat glasses with oil. Preheat the 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. Fold in frozen blueberries. 1⁄3 fill high into the glasses. Bake in the oven for about 35 minutes.
Simmer the lid and preserving rings in boiling water for 2 minutes. Drain. Remove the cakes. Close jars immediately with preserving rings, lids and clips. Let them cool down.