Keep aside 8-10 raspberries. Combine the rest of the raspberries and chopped white chocolate and corn starch in a bowl. Toss the berries and white chocolate until evenly coated.
120 grams white chocolate, 120 grams raspberries, 1 tablespoon cornstarch
Add the berries and white chocolate to the cake mix and fold until just combined. Spoon the cake batter into the prepared loaf tin. Toss the rest of the raspberries on top of the cake. Sprinkle the granulated or raw sugar over the top.
2 tablespoons raw sugar
Bake for 45-55 minutes or until a cake tester comes out mostly clean. Allow the cake to cool in the tin for 10 minutes before lifting it out and placing it on a wire cooling rack. Allow the cake to cool completely before slicing and serving.
All temperatures stated are for conventional ovens. Decrease the temperature by 20°C/25°F or gas mark 1 for "fan" or air-fryers.
Notes
Raspberries - I used frozen raspberries, do not defrost! Blueberries, boysenberries, blackberries or chopped strawberries would also work well in this recipe.
White chocolate - Highly recommend using a good bar of white chocolate and roughly chopping it. This gives a nice compliment to small white chocolate pieces that melt into the cake batter and chocolate chunks that become slightly caramelised.
Gently fold in the berries and white chocolate, the berries are delicate and need to be treated as such.
If the weather is really warm, keep the raspberries in the freezer right up until they are needed to be tossed with the corn starch.
This cake will keep for several days in an airtight container at room temperature.