Slice bread about 3/4" thick. Place in rimmed baking sheet/pan.
In a medium bowl, whisk together the eggs, milk, baking powder and vanilla. Slowly pour over the bread. Turn bread to coat completely. Cover with plastic wrap and set at room temperature for 1 hour or refrigerate overnight.
Grease a 9x13 pan or baking dish. Place frozen blueberries, sugar, cinnamon and cornstarch in rimmed pan or dish. Stir to coat berries.
Place bread, wettest side up, on berries. Wedge slices in tightly, cutting some pieces to fit if necessary.
Brush tops of bread with melted butter.
Bake in center of preheated 450 degree oven for 25 minutes or until the toast is golden and the berries are bubbling around the sides.
Remove from the oven and sift powdered sugar over the top. (Optional suggestion: Grate nutmeg over the top before the sugar). Let sit for 5 minutes before serving.
Lift toast onto serving plates, then spoon blueberry sauce over the top.