Place the flour, 1 tbsp of the sugar and salt in a large bowl. Whisk to combine. Add the butter and using a food processor, pastry cutter or your chilled fingers cut the butter into the dry ingredients until it resembles coarse meal.
Drizzle in the cold water, tossing the flour mixture as you go to ensure even distribution. Add only as much water as you need to get the dough to come together.
Turn the dough onto a floured surface and form into a ball before flattening into a disc. Wrap the pastry tightly in parchment paper and place in the fridge. Let rest for at least 1 hour.
While the pastry is resting, place the peaches, the remaining sugar and cognac in a large bowl. Toss to combine. Cover with plastic wrap and set aside to marinate at room temperature for 1 hour.
Once the hour is up, add the lemon juice and the cornstarch to the peaches and gently stir to combine. Set aside.
Place the pastry on a well-floured surface. Using a floured rolling pin, roll the pastry out to a rough circle shape measuring a 1/8 of an inch thick. Transfer the pastry to a baking sheet or pizza pan.
Arrange the peach slices in the center of the pastry leaving a 2-inch bare border around the outside. Fold the edges of the pastry towards the center of the galette. Transfer the assembled galette to the fridge. Chill for at least 30 minutes. Preheat the oven to 375°F
In a small bowl, whisk to combine the egg and 1 tbsp of water. Using a pastry brush, paint the pastry lightly with the egg wash. Transfer the galette to the oven and bake for 45 minutes, rotating once.
Take the galette out of the oven and set aside to cool slightly. Place the marmalade in a small bowl and microwave it for 20 seconds. Using a pastry brush, paint a thin layer of the marmalade on the peaches. Adding just enough to make them nice and glossy.
Slice the galette and divide amongst 4-6 plates. Serve with a hearty scoop of Snickerdoodle Ice Cream.