Maybe the butter should be melted? Then the whisk would make sense.
Similar method, though differing amounts of ingredients in Dorie Greenspan's Custardy Apple Squares, which I just made last night. And Dorie's recipe does have a bit of milk, but then it has a lot less butter.
QuoteUse a whisk to combine the eggs, sugar and salt in a large bowl for about 2 minutes, until the sugar has just about dissolved and, more important, until the eggs are pale. Whisk in the vanilla extract, then the milk and the melted, cooled butter. Add the flour mixture into the bowl; use the whisk to form a smooth batter.
Add the apples to the bowl; switch to a flexible spatula and gently fold in the apples, turning the mixture until each thin slice is coated in batter. Scrape the mixture into the pan, smoothing the top as evenly as you can. It will be bumpy; that’s its nature
https://www.washingtonpost.com/recipes/dorie-greenspans-custardy-apple-squares/14940/
