To make the crust: place all dry ingredients ( flour, sugar and salt ) in food processor and pulse few times.
Add chilled butter and pulse until the mixture resembles coarse crumbs.
Drizzle in the ice water, until you will see the dough forming.
Turn your dough onto a surface, lightly dusted with cornstarch. This will help with stickiness without adding more flour to the dough and making it tough later.
Do not knead the dough, but gently form into a round disk.
Place on a large piece of parchment paper or aluminum foil and wrap it in it.
Chill in refrigerator for an hour.
In the meantime, make the filling: place sliced peaches in a medium mixing bowl, sprinkle with flour and sugar.
Gently toss to coat.
Drizzle with vanilla extract. Set aside.
Preheat your oven to 425 degrees and place the rack on the middle position.
Take your dough out of the refrigerator and uncover but keep it on the paper.
Roll it out into a circle slightly bigger than your standard 8" pie dish.
Place the filling in the center, arranging the slices as desired.
Place pieces of the additional teaspoon of butter on top of the filling.
Lift the edges of your crust and overlap. You can slightly lift the paper up and fold the crust, making sure not to cover the filling entirely.
Brush the edges with egg wash and sprinkle with sugar.
Brush the visible peach filling. Note: You will have some egg leftover.
Place the paper with your galette on a baking sheet.
Bake for 18 - 22 minutes, until the edges are nice and golden brown.
Let the galette cool on a baking sheet for few minutes, then carefully slide the paper with it onto a cutting board. Cool additional 15 minutes.
Slice and serve dusted with powdered sugar or vanilla ice cream.