Stir the ground coffee and the flour into the milk, season with a pinch of salt, and cook over low heat, stirring constantly, into a thick paste (about 5 minutes); remove from the heat and let cool.
Beat the egg yolks with the sugar and the butter until fluffy.
Add the cooled coffee paste, and mix thoroughly.
Whip the 3 egg whites into a stiff foam, and fold gently into the coffee base.
Pour into a buttered heatproof dish.
Bake in an oven preheated to 190 °C for about 20 minutes, until risen with a slightly golden top.
Serve immediately, while hot. (The quantities can be doubled in the same proportions.)
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.