Combine flour, 1 cup of sugar, baking powder, and salt in a medium bowl. Add butter to flour mixture and toss. Using a fork or pastry cutter, blend in the butter to create a crumbly texture. Whisk the egg slightly, and stir it into the mixture. Press half of the dough into a greased 9x13 baking pan.
In a separate bowl, stir together your remaining ½ cup sugar and your cornstarch. Add the sliced peaches and gently mix. Spoon peaches over the pressed crust. Crumble your remaining dough over the peach mixture.
Bake at 375 for 45-55 minutes. Top should be slightly brown.