Preheat the oven to 350F/180C and spray eight 4oz jelly jars with cooking spray. Place the jars into a 13x9-inch pan.
Place the lemon zest and juice, melted butter, flour, and 3 tbsp of sugar into a medium-sized bowl and whisk until you have a smooth paste. Set aside.
Place the egg yolks into a medium-sized bowl and the egg whites into a large bowl; a metal one is preferable for the egg whites.
Add the milk to the egg yolks and whisk until smooth. Gradually pour the milk mixture into the lemon/flour mixture, whisking as you go, until smooth and well combined. Set aside.
Beat the egg whites on high speed, with a hand mixer or your stand mixer fitted with a balloon whisk, until they just hold peaks.
Gradually add the remaining sugar, while continuing to beat the egg whites. Keep beating until the egg whites are glossy and hold their peak; this will take around 5 minutes.
Using a metal spoon, add 1/4 of the egg white mixture into your milk/egg mixture and gently fold until combined. Repeat another 3 times until all of the egg whites are incorporated. Be very gentle when folding, you want to keep as much air in as possible and end up with a really light, airy, and somewhat runny batter.
Divide the mixture between your jars, using about 1/3 cup of batter for each jar. They should fill up to the lowest rim line of your jars.
Pour boiling water into the 13x9 inch pan making sure the water level is just below halfway up the jars.
Place in the oven for 15-20 minutes, until risen, lightly golden around the edges and they feel springy to the touch.
Cool in the pan for 15 minutes before serving with whipped cream and grated chocolate.