1. Preheat an oven to 160 degrees Celsius fan, or equivalent, and line a 18cm x 28cm baking tin with parchment paper (or use a 20cm square).
2. Using an electric whisk, cream together the butter and sugar until pale and fluffy. Add vanilla. Gradually add the eggs, one at a time.
3. Sieve the flour and baking powder together into a bowl then mix the sieved ingredients into the batter. If the batter is very stiff after adding the flour, mix in the boiling water to loosen the consistency, if required.
4. Transfer half the batter onto the base of the lined baking tin. Toss two thirds of the blackberries in a teaspoon of flour to lightly coat them before scattering them over the batter in the tin (tossing them in flour stops the berries sinking to the bottom).
5. Cover the blackberries with the remaining batter.
6. Bake on the middle shelf of the preheated oven for 35-40 minutes until risen in the centre, or until a skewer inserted into the centre comes out clean.
7. Allow to cool in the tin for 10 minutes before upturning the sponge onto a wire rack and peeling away the baking parchment. Allow the cake to cool completely before topping with cream cheese icing.
8.To make the icing, beat the butter and cream cheese together until lump free. Add the sieved icing sugar in three batches, beating till smooth, lastly mix in the lemon zest.
9. To serve, spread the icing over the top of the cooled cake and scatter with the reserved fresh blackberries.
Variation: Serve with blackberry coulis, made by simmering 150g blackberries with 25g sugar and half a teaspoon of vanilla extract, blitzing, then sieving to a smooth purée.