Preheat & Prep
Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.
Prepare the Filling
In a bowl, toss strawberries with sugar, cornstarch, lemon juice, and vanilla. Set aside.
Make the Crust
In a mixing bowl, whisk gluten-free flour, cocoa powder, sugar, and salt.
Cut in cold butter with a pastry cutter or fork until crumbly.
Form the Dough
Add cold water 1 tablespoon at a time until the dough just comes together.
Shape into a disc, wrap, and chill 30 minutes.
Roll the Dough
Roll dough between two sheets of parchment into a 10–12 inch circle.
Transfer to the prepared baking sheet.
Assemble the Galette
Spoon strawberry filling into the center, leaving a 2-inch border.
Fold edges over the filling, pleating gently.
Finish
Brush crust with egg wash if using and sprinkle lightly with sugar.
Bake
Bake 40–45 minutes, until the crust is set and the filling is bubbling.
Cool
Let cool 15 minutes before slicing.