Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Start by lining the tin. You will need a 23 x 33 cm / 9 x 13 in traybake tin which you will divide into three sections.
Fold a piece of tin foil over several times into a thick piece the same width as the tin and either the same height or slightly higher. Repeat this so that you have two thick strips of tin foil.
Place these strips crosswise in the tin, dividing it into three even sections. Take a long piece of tin foil (about 75 x 23 cm) and use it to line the tin, going over the top of the two foil strips to hold them in place.
Brush the foil with melted butter and place a long strip of baking parchment (about 75 x 23 cm) over the top, folding it over the tin foil strips. (If you have some of that 2-in-1 foil and parchment then use that instead). Try and line the tin as neatly and evenly as possible so that you don't have to trim the cakes too much later.
To make the cake, start by weighing the bowl that you are going to be making the batter in; take a note of the number.
Sift the plain flour, cornflour, baking powder, bicarbonate of soda and salt into the bowl. Then whisk in the sugar.
In a large jug, whisk together the milk, yogurt, melted butter, oil and vinegar.
Pour the wet ingredients into the dry and stir with a balloon whisk until no dry lumps remain.
Weigh the bowl again. Subtract the weight of the empty bowl and divide the resulting number by three. This tells you how much batter you need to weigh out into each bowl.
Divide the batter between three bowls, weighing out the amount from your calculation into each.
Stir a teaspoon of vanilla extract into one bowl, into another stir a teaspoon of lemon extract (or vanilla) and a drop of yellow food colouring, and into the third stir a teaspoon of raspberry extract (or vanilla) and a drop of pink food colouring. You really do only need a teeny bit of food colouring, don't be heavy handed with it!
Pour each colour of batter into a separate section of your prepared tin and spread them level.
Bake for 20-25 minutes until a skewer inserted into the centre comes out clean. Allow to cool in the tin for 15 minutes then carefully turn them out onto a wire rack and leave to cool completely.
Once the cakes are cold, level the tops of the pink and yellow layers (but not the white one), and trim the sides so that they are all exactly the same size.
To make the buttercream, whisk the butter, sifted icing sugar and vanilla extract together until light and fluffy.
Spread half of the buttercream over the yellow cake layer and place the pink layer on top. Spread over the rest of the buttercream and top with the white cake layer. Store in an airtight container.