Grate lotus root and gently squeeze out liquid. (Do not squeeze completely dry but the lotus root pulp should still be moist enough.)
Cream butter with half the sugar and salt until creamy and light. Beat in egg yolks two at a time, beating well after each addition.
Sift flour and baking powder together and fold into creamed mixture. Stir in essence and grated lotus root until combined.
Beat egg whites until firm peaks form, then add in remaining sugar gradually. Whisk until mixture is glossy and thick. Fold meringue into the creamed lotus root mixture. Add blackcurrants to mix.
Transfer mixture to a greased 20cm square or round cake pan. Bake in preheated oven at 180°C for 70 minutes or until a skewer comes out clean when tested. Allow cake to sit in the pan for 5-10 minutes before turning out onto a wire rack.