The vanilla cupcake recipe from Joy of Baking is a standard buttercake recipe.
Creaming the butter and sugar allows air to be incorporated into the mixture. The eggs are added, one by one, and serve as binders to hold the structure together. Folding in the flour and adding the liquid maintains the air. Heating it starts the chemical/physical reactions.
The method CakeJournal used [whisking the eggs and sugar and melting the butter with the milk] is reminiscent of the Hot Milk Sponge technique.
Beating the eggs and sugar until thick and light aerated the mixture. Folding in dry ingredients, and pouring in the liquids lightly and quickly, means the air is not lost. The procedure produces a soft light sponge .
The vanilla cupcake recipe from Joy of Baking works both ways because there is a smaller amount of butter into the recipe. Once the amount of butter or liquid increases, the Hot Milk Sponge technique would be less likely to succede.
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