Preheat the oven to 180°C/ 160°C fan/ 35°0F/ Gas Mark 4. Line a 20.5 x 30cm (8 x 12in) tin with nonstick baking paper. Cream the butter and sugar together in a bowl until fluffy. Add the eggs one at a time, then the vanilla essence, beating until combined. Sift and fold in the baking powder and flour, then stir through the blueberries. Spoon the mixture into the cake tin. Bake for 30 minutes until risen and golden or until inserting a skewer in the middle comes out clean. Leave the cake to cool completely in the tin. Meanwhile, make the icing. Cream the icing ingredients together until smooth. Chill for at least 30 minutes while the cake cools. Spread the icing over the cake and scatter with extra blueberries. To store, keep chilled or in a cool room to prevent the icing softening. Tips: The sponge freezes very well if, say, you want to eat half straight away and half at a later date - simply half the icing quantities each time. It is very important that both the butter and the cream cheese for the icing are at room temperature so that they blend together well without forming any lumps. Once it has been spread on the cake and chilled, the butter in the icing will help it set very firm, so it will stand up to being packed in a lunchbox or picnic basket well. In order to enjoy optimum flavour and quality, frozen items are best used within 3 months of their freezing date.