Preheat oven to 180°C. Spray an 18 x 28cm slab [slice] pan with oil [any kind]. Line the base and sides with baking paper, allowing the paper to hang over the edges of the pan.
Use an electric beater to beat the butter, caster sugar, strawberry essence and vanilla extract in a bowl for a few minutes until pale and creamy. Add eggs, beat some more.Add the flour and milk. Stir with a wooden spoon until mixed thoroughly.
Divide the mixture into 3 bowls. Add the pink food colouring to one of the bowls and mix well. Add the cocoa powder to the mixture in the other bowl and mix well. Drop dollops of plain, pink and chocolate batter into the prepared pan, alternating as you please. Once all the batter is in the pan, use a fork to gently swirl the tops of the batter and blending into each other.
Bake for 20-25 minutes in the pre-heated oven. Wait for it to cool completely before cutting into slices. Serve dusted with icing sugar.