In a medium sized saucepan, add the cherries, the brown sugar, the cognac, the lemon zest and juice, and the cornstarch and stir. Bring to a boil and let simmer over a medium heat until it reduces and thickens, but it is still a little liquidy (it will thicken when it bakes)

While the cherry filling is simmering, prep the dough making sure to stir the filling occasionally

In a medium sized bowl, mix the flour, salt, and baking powder

In a separate medium sized bowl, or preferably in a stand mixer bowl, add the butter and regular sugar and mix together using a paddle attachment on a medium-high speed setting until they become fluffy

Scrape the butter-sugar mixture down and add the 1 egg and 1 yolk. Mix together on a medium- low speed

Mix in the flour mixture into the butter mixture slowly and in two batches until a dough ball begins to form. Separate the dough ball into two equal parts

Place both dough balls between two separate sheets of either wax or parchment paper. Roll one of the dough balls out to a little over 11 inches in diameter. Set it aside. Roll the other dough ball to a little over 9 inches in diameter. Keep the rolled out dough sandwiched between the paper sheets. Stack them on top of one another on a baking tray and refrigerate until they firmed and are flexible and can be molded, between 30 minutes to an hour

The cherry filling should be reduced by now so let it rest while the dough is in the refrigerator

Preheat the oven to 375 degrees

Take some butter and grease up a 9 inch cake pan, or better, a springform (cheesecake) pan, at the base and sides. Add a piece of wax or parchment paper to the bottom and grease it as well

Remove the dough from the refrigerator and let it rest at room temperature for about 5 minutes. Take the largest dough round and place it in the pan gently, tapping all around and forming the side

Add the cherry filling

Take the second dough round and gently place it on top. Once it has sealed, using a small paring knife, fold the sides over and gently tap down with your hands, sealing the gateau and forming a crust

Using the remaining egg, mix it in a bowl with a few drops of milk and make an egg wash. Before coating the gateau with the egg wash, using a fork, gently make a pattern of crisscrossed lines across the top and then coat it with the egg wash

Place it in the middle rack of the oven for about 35 minutes or until golden brown

After it has baked, remove and let cool for a few minutes. Then, using a paring knife again, dislodge the gateau from the sides of the pan. Transfer it onto a plate or serving dish and let cool completely

Serve with ice cream or eat as is