Preheat the oven to 350°F (180°C). Butter a 10-inch (25-cm) pie plate.
Heat the 2 tablespoons butter in a frying pan or skillet. Add the apple slices and cook until soft but still holding their shape.
Spread the cooked apple slices evenly in the pie dish.
In a large mixing bowl, whisk together the eggs, flour, sugar, salt, milk, and vanilla extract until well blended.
Pour the liquid filling over the apples in the pie dish.
Bake the flognarde for 45 minutes until well puffed and golden.
Allow the flognarde to cool, dust with powdered sugar, and slice.