Preheat oven 180 degrees. In a bowl, combine sugar, melted butter and salt. Mix everything well. Add oil and mix until smooth for about 3 minutes.
Add one egg at a time and stir well. Then, add the yellow lemon zest, lemon juice and mix well. Add kefir and mix.
Add soda and vanilla powder to flour. Pour the resulting flour in two stages into a bowl with liquid mixture, stirring for 20 seconds each time.
Add 1 tbsp of flour to the blackcurrants and mix well. Then, add berries to the mixture. Pour the mixture into the moulds. Bake for 25 minutes in a heated oven at 180 degrees.