Preheat your oven to 350°F (175°C). Spray three 8-inch round cake pans with nonstick cooking spray.

In a medium mixing bowl, whisk together gluten-free flour blend, cornstarch, baking powder, and kosher salt. Set aside.

In a measuring cup, combine milk, heavy cream, and vanilla extract, stirring gently. Set aside.

Cream together softened butter and granulated sugar on medium speed until fluffy, about 3 to 4 minutes. Add eggs one at a time, mixing thoroughly.

Gradually add the dry mixture and prepared milk mixture to the creamed butter, alternating between the two until just combined.

Let the batter rest for 15 to 20 minutes to allow the gluten-free flour to hydrate.

Pour the rested batter into the prepared cake pans and bake for 45 to 50 minutes. Check for doneness with a toothpick.

For the caramel, combine heavy cream, brown sugar, light corn syrup, and flake salt in a saucepan. Heat over medium-low, stirring constantly until sugar dissolves.

Cook the caramel until it reaches 234°F (112°C) using a candy thermometer. Add vanilla extract, then cool slightly before using.

Assemble the cake by placing one layer on a platter, spreading caramel between layers, drizzling caramel on top, and sprinkling with flake salt.