If you are using a special Battenberg tin that comes with dividers to make four strips of sponge, grease and line the tin with butter and baking paper.
Alternatively, you can make your own: cut a 20 x 28cm rectangle of parchment-lined foil, set it paper-side up on the worktop and fold it in half along the long length.
Open it out, then push the centrefold upwards to make a pleat 4cm high.
Crease it firmly, then press the parchment-lined foil sheet with the centre pleat onto the base of greased tin to line it – the pleat will run down the centre of the tin to divide it into two separate sections each 20 × 10cm. Preheat the oven to 180°C/160°C fan/350°F/Gas 4.
To make the sponge, put the butter into a mixing bowl or the bowl of a food-mixer and beat until creamy with a wooden spoon or the whisk attachment.
Scrape down the sides of the bowl and whisk in the sugar a couple of tablespoons at a time.
Scrape down the sides of the bowl where the mixture has splattered up, then beat really well for a couple of minutes until the mixture is light and fluffy.
Scrape down the sides of the bowl again to get any stray mixture.