Grease and line 8″/20-cm square baking pan with parchment paper. Fold a larger piece of aluminium foil into half to create 2 layers, top with 6-7 pieces of kitchen towels, wrap the bottom of the baking pan with the aluminium foil.
Preheat oven to 150°C/300°F.
Combine butter and evaporated milk, heat up over medium-low heat until butter has melted.
Sift in cake flour, mix until combined.
Combine 1 egg and 7 egg yolks, add to batter gradually, mix until smooth.
In a clean mixing bowl, whisk egg whites on low speed until foamy. Add in salt and lemon juice, whisk until small bubbles are formed. Add in sugar gradually, whisk on low speed until soft peak (meringue is shiny with arch shape).
Add meringue to egg yolk batter in 3 batches, mix gently with hand whisk. Fold gently until well mixed with spatula. Tap mixing bowl a few times on countertop to burst large bubbles. (Reminder: Do it quick but gentle to avoid deflating the meringue. Scrape the bottom of mixing bowl with spatula and fold over, repeat folding motion until just well mixed.)
Pour the batter into lined baking pan, tap a few times to break large bubbles.
Put baking pan with batter into a larger pan, then fill the larger pan with plain water, about 1″ high, for water bath.
Bake at 150°C/300°F for 1 hour 50 minutes, until fully cooked. (Reminder: If your Cotton Sponge Cake does shrink when out of oven, then your oven temperature is too hot. Try reducing baking temperature to 140°C. The cake should rise slowly during baking to prevent it from over-rising then collapsing when out of oven.)
Drop the baking pan from 1 foot high onto countertop to minimise shrinkage.
Unmould Cotton Sponge Cake and cool completely on wire rack (at least 6″ from countertop).