Preheat the oven to 180°C/gas mark 4. Grease a 2lb loaf tin and line with baking parchment.
Put half the butter in a bowl with the light brown sugar and beat with a handheld electric whisk until light and fluffy. Beat in one egg at a time with 1 tbsp flour. Use a wooden spoon to stir in the food colouring, yogurt and vanilla extract. Fold in the remaining flour with the cocoa powder until smooth.
Spoon the mixture into the prepared loaf tin and bake for 50-55 minutes, until a skewer inserted into the middle comes out clean. Remove from the tin and cool on a wire rack.
Put the remaining butter in a bowl and use a handheld electric whisk to beat until smooth, then beat in the icing sugar. Add the soft cheese and beat until thick and pale. Spread over the cooled cake, then top with the defrosted berries and spoon over the reserved juices.