Preheat the oven to 180 C / Gas 4. Grease two 20cm (8 in) round cake tins.
Cream together the butter, sugar and vanilla extract until light and fluffy.
Sift together the flour and baking powder.
Gradually add the flour and eggs to the butter mixture, adding the flour in three additions along with an egg each time. Whisk together until well combined.
Add equally into the two baking tins and bake for 25 minutes. It should be ready when a skewer comes out clean from the centre of the cake.
Let them set in the cake tins for 5 minutes. Then, put them on a cooling rack to cool.
Whip the double cream into peaks along with the icing sugar. It should be smooth and stiff.
Slice enough strawberries to cover the top of one cake. Add a generous layer of jam and cream and put strawberries on top.
Sandwich together the two cakes.
With the remaining cream, add a thin layer to the whole of the cake, use a spatula to make it smooth.
Slice the remaining strawberries in half. Melt 150g of dark chocolate and dip the strawberry halves in. Wait for the chocolate to harden.
Melt the white chocolate and drizzle over chocolate covered strawberries. Arrange nicely on the cake.
Grate the lemon zest in three long strips. Make sure they are long enough to go around the circumference of the cake. Add to the sides of the cake in a stripe pattern (as shown on picture). This step is quite tricky, alternatively you could use ribbon.
Using a blunt knife, scrape across the edge of the extra dark chocolate (make sure it is in a bar, not melted). This should create extra thin chocolate gratings. Sprinkle over the cake.
Arrange the blackberries in the middle and edge of the cake.