2) In a standing mixer fitted with the paddle attachment, cream the butter at low speed until smooth, about 1 min. Slowly add the sugar and mix until just incorporated. Raise the speed to high and mix until light and fluffy, about 10 mins. Occasionally turn the mixer off, and scrape the sides of the bowl down with a rubber spatula.
3) Meanwhile, sift the flour, baking powder and salt together into a large bowl. In a small bowl, whisk together the milk and vanilla. Reduce the mixer speed to low; alternately add the flour mixture and milk mixture, waiting for each to be fully incorporated before adding the next (scrape the bowl down occasionally). Raise the speed to medium and mix briefly until a smooth batter is formed. Transfer the batter to a large bowl.
4) Thoroughly clean the bowl of the mixer and place the egg whites inside. Using the whisk attachment, whip the egg whites on high speed, until stiff peaks are formed. Working in 3 batches, using a rubber spatula, fold the egg whites into the cake batter, until just incorporated.
5) Divide the cake batter evenly between the prepared cake tins. Place the tins on the middle oven rack evenly spaced apart. Bake, rotating the pans halfway through the cooking, until golden brown and a toothpick inserted in the centre of the cakes comes out clean, about 30 mins.
6) Remove the cakes from the oven and run a knife around the edges to loosen them from the sides of the tins. One at a time, invert the cakes onto a plate and then re-invert them onto a cooling rack, rounded-sides up. Allow to cool completely.
7) To add the icing, place one cake, rounded-side down, in the middle of a rotating cake stand. Using a palette knife or offset spatula spread about half the icing all over the cake to make a 0.5–1cm deep layer. If using the vanilla icing, reserve about 250g to 500g for colouring and decorating. Carefully set the other cake on top of the icing, rounded-side up, and spread all over with the remaining icing.
8) If adding colouring icing, place the reserved icing in a medium bowl and stir in the food colouring, little by little, until the desired colour is reached. Transfer the coloured icing to a pastry bag with a small, round or fluted tip, or a plastic baggie with a small portion of a corner cut off.
9) Using the coloured icing, creatively write "Happy Birthday" to whomever and decorate the edges in fringes and whirls, if desired. Serve immediately with lit candles or store at room temperature or in the refrigerator covered with a cake or pie lid. (If storing the cake in the refrigerator, remember to remove it several hours before serving, to bring it to room temperature.)
For the icing:
1) In a small bowl, whisk together the milk, vanilla, lemon juice, and zest. (Omit the lemon juice and zest if making chocolate icing.)
2) In a mixer, cream the butter at low speed until smooth, about 1 min. Slowly add the sugar and mix until just incorporated. Raise the speed to high and mix until light and fluffy, about 10 mins.
3) Reduce the speed of the mixer to low. Add the milk mixture in 3 batches, waiting for each batch to be fully incorporated before adding the next (scrape the bowl down occasionally). Raise the speed to high and mix briefly until fluffy.
4) If making chocolate icing, heat a small pot of water over high heat to boiling point, then reduce to a simmer. Put the chocolate in a heatproof bowl and place over the simmering water until the chocolate is 2/3 melted, taking care it doesn't burn on the sides of the bowl. Remove from the heat and stir to melt the remaining chocolate. Add the chocolate to the prepared icing.
5) Use the icing as desired – it may be stored, covered, in the refrigerator for 3 days.