Preheat the oven to 190 °C / 375 °F. Put the eggs together with sugar in a glass bowl over a saucepan of simmering water. Using an electric mixer, whisk mixture over the heat for about 5 minutes, or until thick, pale and doubled in volume. In a separate bowl combine baking soda, flour and pinch of salt. Sift half of dry mixure into the egg mixute and very gentle fold it through. Sift over the rest of the dry mixture. Again, fold it in gently. Add melted (but cold) butter to the mixture. Grease cake molds with butter, then dust with flour. Divide the mixture between both cake molds. Bake for 20 minutes at 190 °C / 375 °F, until golden and springy to touch. Remove from the oven and let cool completely on a wire rack if possible.