With the remaining sugar, add the butter to it and beat it with a mixer until it's light (it will still be grainy). Add egg yolks one at a time, mixing well after each. Blend in the zest. Mix flour and salt into your batter. Now add your milk and lemon juice and give it all another mix.
In another bowl (be sure it's super clean, egg whites don't like grease of any kind) mix your egg whites until they're frothy soft peaks. Sprinkle the 2 Tbsp of reserved sugar over them and continue to beat the whites until they form stiff peaks.
Gently fold your whites into the batter. Start by pouring the whites on top of the batter. Use a spatula and stir the batter around the bowl, then gently lift up and turn it over the whites. You're looking to combine the two elements without crushing all those little bubbles you just built into your whites. Eventually it will all be the same colour and texture. Once it is, stop
Place your ramekin into a high-edge roasting pan. Ladle your batter into the ramekin. Now carefully pour water into the roasting pan so that it comes up about halfway the ramekins.
Pop them in a hot oven for 40 to 45 minutes. Watch it closely at the end - you want it to be golden but not brown. It will be a firm but soft sponge on the top. Remove from the oven and allow it to cool for a few minutes before shaking some confectioner's sugar through a sieve over the top. Use a large spoon to scoop out a portion, making sure you go right to the bottom for some of the delicious pudding sauce.