Preheat the oven to 180°C/Fan 160°C/Gas 4. Grease a 30 x 23cm traybake tin and line with non-stick baking paper.
Measure all the traybake ingredients, except the strawberries and white chocolate, into a large bowl and beat with an electric mixer until light and fluffy. Stir in the strawberries and melted chocolate.
Turn the mixture into the tin and level the top. Bake in the preheated oven for 30–35 minutes, or until the cake has shrunk a little from the sides of the tin and springs back when pressed in the centre with your fingertips. Leave to cool in the tin.
To make the icing, measure the butter, icing sugar, milk and vanilla into a bowl. Whisk until pale and creamy. Spread over the traybake and sprinkle with the strawberries.