Preheat the oven to 325°F and wrap a 9-inch springform pan with two layers of foil on the outside. In a medium bowl, stir together the gluten free graham cracker crumbs, melted butter, brown sugar, and fine sea salt until evenly combined.
Press the mixture firmly into the bottom of the pan. Bake for 10 minutes, then remove and let cool. In a large bowl, beat the cream cheese and granulated sugar on medium speed for about 4 minutes until very smooth and free of lumps. Add 1 cup of the dulce de leche and beat until completely incorporated.
Mix in the cornstarch and vanilla on low speed. Add the eggs one at a time, beating on low after each addition just until the yolk disappears. Fold in the sour cream by hand with a spatula until smooth.
Pour the filling over the cooled crust. Warm the remaining 1/4 cup of dulce de leche in the microwave for 10 seconds, then drop spoonfuls across the surface of the filling. Use a toothpick or skewer to gently swirl it through the batter in a decorative pattern.
Set the springform pan inside a roasting pan and pour in enough hot water to come halfway up the sides. Bake for 60 to 65 minutes, until the outer edges are set and the center still has a gentle wobble when you lightly nudge the pan.
Turn off the oven, crack the door open, and let the cheesecake cool inside for 1 hour. Remove from the water bath, peel off the foil, and refrigerate for at least 6 hours or overnight.
Whip the heavy cream and powdered sugar together until stiff peaks form. Spread it over the chilled cheesecake. Drizzle with dulce de leche and finish with a pinch of flaky sea salt before serving.