Heat oven to 180C/fan 160C/gas 4. Butter, then line, a shallow rectangular tin, approx 25cm x 22cm. Melt the butter in a pan, then cook gently for about 5 minutes, until the white solids turn golden and the butter smells biscuity and sweet. Cool for 5 minutes, then add half the white chocolate and set aside for a couple of minutes to melt. Stir until smooth.
Use electric beaters to whisk the muscovado sugar and eggs together for about 2 minutes until thick and pale. Tip in the flour, salt, cooled chocolatey butter and vanilla, then fold in with a metal spoon or spatula until even.
Pour the mix into the tin, then scatter with the rest of the chocolate and the raspberries. Bake for 40-45 minutes or until risen all over, with a dark golden crust. Cool completely in the tin, then cut into squares.