Preheat the oven to 190℃ (170℃ fan)/375°F/gas mark 5. Line the base and sides of a 900g (2lb) loaf tin.
Set aside a few raspberries to decorate then roughly mash the remaining raspberries together with the raspberry jam in a small bowl and set aside. Cut 100g (3½oz) of the chocolate into small chunks, set aside.
In a mixing bowl, beat the butter and sugar together until light and fluffy. Add the eggs one at a time beating well after each addition.
Fold in the white chocolate chunks and flour.
Spoon about one-third of the sponge mixture into the bottom of the tin. Cover with half the raspberry mixture. Top with another third of the sponge mixture followed by the remaining raspberry mixture.
Finally top with the remaining sponge mixture and use a blunt knife or a skewer to gently ripple the two mixtures together.
Bake for 1 to 1 1¼ hours or until a skewer inserted into the centre of the cake comes out clean. Allow to cool in the tin for a few minutes before transferring to a wire rack to cool completely.
Melt the remaining 25g (1oz) chocolate in a bowl over a pan of hot water and drizzle over the top of the cake, top with the reserved raspberries and allow the chocolate to set before slicing to serve.
Nutrition information is approximate and is meant as a guideline only.