Add ¾ cup milk to bowl of a stand mixer fitted with dough hook attachment; sprinkle yeast over top. Whisk together 4 tablespoons melted butter, 2 tablespoons honey, 1 egg, and 1 egg yolk; add to bowl with milk and yeast and mix with a wooden spoon to combine.
Whisk together 1½ cups all-purpose flour, 1½ cup whole-wheat pastry flour, ¼ cup cornstarch and ¾ teaspoon salt; add to bowl with wet ingredients and mix on low until a dough comes together, 1–2 minutes. Increase speed to medium-low and knead until a smooth, sticky dough comes together, about 8 minutes.
Transfer dough ball to an oiled bowl, cover bowl loosely with a kitchen towel and let rise for 1½–2 hours, or until doubled in size.
Meanwhile, grease a 9-inch round or square cake pan and line with parchment paper; grease parchment paper.
Transfer dough to a floured surface and roll out into a 14x9-inch rectangle; long side parallel to counter’s edge.
Brush 1 tablespoon melted butter over dough, leaving ¼-inch border on farthest edge.