Make the cornets: Preheat the oven to 375°F. Line a cookie sheet with parchment paper. Make your molds. Take a sheet of aluminum foil and mold it into a cone shape about 3-4 inches long and about 1-1/4 inches in diameter at the top of the cone. Make 4 of these cone-shaped molds.
In a small saucepan, melt the butter over medium-low heat, then remove from heat. Whisk in the sugar. Add the vanilla and egg white; whisk for 1 minute. Add the gf flour and xanthan gum and whisk until smooth. The batter will be fairly liquid.
Spoon 1-2 heaping tablespoons of the batter onto the prepared cookie sheet, making only 4 cookies at a time. Using the bottom of the spoon spread the cookies in a circular motion until the cookie is about 5 inches in diameter. Bake the cookies for 10 minutes or until golden (they will still be very soft). Remove the cookies from the oven and let rest on the cookie sheet for 1 minute before molding (if they are molded too soon they will crack). Working quickly, roll each warm cookie around the aluminum cone-shaped molds. Set the cookies on a rack to let cool, seam side down. When cool, carefully remove the aluminum cones. Repeat this process with the remaining batter.
Whip the cream with 2 tablespoons of sugar until stiff. Transfer the whipped cream to a pastry bag. Place each cornet onto dessert plates. Pipe the whipped cream into the center of each cornet. Place fresh berries in and around the cornet. Using a sifter or a fine mesh sieve sprinkle powdered sugar over the top and around the plate. Serve immediately.
The cookies can be made ahead of time. Let cool completely and seal them in a container with a tight fitting lid.