First weigh 4 eggs in their shells, then weigh equal amounts of butter, sugar and flour.
Cream the butter and sugar until light and fluffy.
Beat in the eggs one by one. If the mixture starts to curdle, add a spoonful of flour.
Sieve the flour and baking powder and throw any bran remaining in the sieve into the compost bin.
Gently stir the flour into the sponge mixture, followed by the kefir until everything is just combined.
Divide the mixture between two 22cm round silicone moulds or lined baking tins and bake at 180℃ (350℉, Gas 4) for 30 minutes or until an inserted skewer comes out clean.
Leave in the moulds to cool for about 10 minutes, then turn out onto a wire rack to cool completely.
Sandwich the cakes with strawberry jam, then dust with icing sugar or caster sugar and serve.
Notes
Please note: calories and other nutritional information are per serving. They're approximate and will depend on serving size and exact ingredients used.