Preheat your oven to 350°F (175°C). Line three 8-inch round cake pans with parchment paper and spray with non-stick cooking spray.

In a large bowl, whisk together Kim's gluten-free flour blend, cornstarch, baking powder, and kosher salt until evenly combined. Set aside.

In a measuring cup, mix together milk, heavy cream, and vanilla extract until well blended. Set it aside.

Cream the softened butter and granulated sugar together in a bowl for 3 to 5 minutes until light and fluffy. Add eggs one at a time, mixing well after each addition.

Gradually alternate adding the dry mixture to the creamed butter and sugar, followed by the wet ingredients, starting and ending with the flour blend. Mix until just combined.

Pour the batter evenly into the cake pans, and bake in the preheated oven for 45 to 50 minutes.

To prepare the caramel, combine heavy cream, light brown sugar, light corn syrup, and flake salt in a medium saucepan. Stir over medium-low heat until dissolved, then bring to a gentle boil.

Cook the caramel, stirring constantly until it reaches 234°F (112°C). Remove from heat and stir in vanilla extract. Let it cool slightly before using.

Once the cake layers are cooled, place the first layer on a cake stand. Spread ½ cup of the cooled caramel frosting over the top. Stack the second layer and repeat, pouring remaining caramel over the final layer.

Finish by sprinkling flake salt on top if desired. Serve at room temperature and store covered for 2-3 days.