Put the butter, golden syrup and sugar in a saucepan and heat gently until the butter has melted. Remove from the heat and stir in the pineapple, sultanas and mixed spice. Leave to stand for 10 minutes, then stir in the bicarbonate of soda.
Place the flours in a large bowl. Stir together the eggs and soured cream, then add to the butter mixture and beat well to mix. Pour into the dry ingredients and stir well to mix.
Turn the mixture into a greased and lined 28 x 24 cm (11 x 9¼ inch) shallow baking tin or roasting tin (see
Make the frosting. Beat together the mascarpone cheese and butter in a bowl until smooth. Stir in the icing sugar and lemon juice. Spread over the top of the cake, then cut into squares.