Prepare the bilberries. Wash the bilberries removing any leaves, stalks and spoilt fruit. Place onto a double layer of kitchen roll to dry.
Preheat the oven to 180℃ / Fan 160℃ / 355℉ / Gas 4. Prepare the cake tin. Grease and fully line the cake tin with grease proof paper.
Make the sponge batter. Place the soft butter and sugars into a good sized bowl and beat together with a wooden spoon or electric beaters until very pale and fluffy. Add the salt and lemon zest. Beat again to combine. Gradually add the beaten egg a little at a time, beating well after each addition, (see note b below). Sieve the flour, ground almonds and baking powder into the mixture. Use a spatula, large metal spoon or your hand with your fingers splayed to fold this in gently. Add a little milk and gently mix in until the batter has a dropping consistency.
Fill the cake tin. Place the prepared bilberries into the bottom of the cake tin, ensuring there is an even layer. Spoon the prepared batter onto the bilberries. Spread the batter out gently to level, aiming not to disturb the bilberries. Use the back of a spoon to make a slight indentation to the centre of the batter. This will help it bake level.
Bake. Place the cake tin in the centre of the oven and bake for about 45 – 50 minutes until golden brown, slightly pulling away from the sides and a cake skewer inserted in the centre comes out clean. You may need to rotate the cake tin after 35 – 40 minutes of baking. Once baked, remove from the oven and place onto a cooling rack.
Turn out the cake. Allow the cake to cool for 5 minutes before removing from the tin. Invert a cooling rack on top of the cake tin. In one movement, swiftly turn the cake tin and cooling rack upside down. Remove the cake tin and greaseproof paper. The bilberries will now be the top of the cake. Allow the cake to finish cooling.
Enjoy, with either on its own or served with ice cream, cream, yogurt or even warm with some custard.