Make the Glaze:
In the same bowl, whisk the flour with the sugar, baking powder and salt. In a small bowl, mix the eggs with the milk and vanilla. Add the mixture to the dry ingredients along with the melted butter and whisk until smooth. Scrape the batter over the apples and spread evenly. Bake the cake in the lower third of the oven until golden and springy and a toothpick inserted in the center comes out clean, about 1 hour and 15 minutes.