Beat the butter and sugar together with electric beaters until pale and fluffy
Crack the egg into a jug, beat with a fork, then add to the creamed butter & sugar a little at a time, beating well between additions. This helps to prevent curdling
Once the egg has been incorporated stir in the lemon zest & juice and a small dab of yellow colouring if using
Sieve the flour and baking powder into a bowl, then fold half into the cake batter, followed by the remainder. Fold into the flour until fully blended, but take care not to overmix the batter
If using the specialist Battenberg tin, divide the lemon cake batter between 2 sections
If using the regular oblong tin, divided in the centre with parchment, spoon the batter into one half of the tin