Add the icing sugar and beat again until combined. Finally, add the lime juice and beat again. Pipe or spread the buttercream onto the loaf cake, then sprinkle over the blueberries and lime zest.
‘This delicate lemon and blueberry sponge ... Step 5: To make the buttercream, beat the butter until smooth and creamy, then gradually beat in the icing sugar and vanilla until light, smooth ...