Preheat oven to 350 degrees and spray three 8” baking pans with nonstick cooking spray. In the bowl of a standing mixer using the paddle attachment, combine organic brownie mix, brown sugar, flour, baking powder, baking soda and ¼ teaspoon of salt (from the salt packet). Mix on low until ingredients are combined.
In a separate bowl, combine eggs and egg yolk, Greek yogurt, milk and oil. Whisk until combined, then pour mixture into the dry ingredients. Mix together on medium speed until just combined. Pour in hot water and blend one minute more; batter will be thin.
Evenly divide batter into prepared pans. Bake 25-30 minutes, or until a toothpick inserted in the center of each cake comes out clean. Cool cakes 15 minutes in pans before removing and cooling completely on a wire cooling rack.
To make the frosting, cream the butter on medium speed until smooth and fluffy; about 3 minutes. In a separate bowl, whisk together the organic frosting mixes and cacao powder. Slowly add powdered sugar mixture and blend until incorporated, about 2 minutes. Add vanilla, salt, milk and 2 tablespoons of salted caramel. Beat another 3 minutes; add additional milk one tablespoon at a time, if necessary.
To assemble, place one layer of cake on a large plate or cake stand. Spread a layer of frosting over the top, then add 3 tablespoons of salted caramel. Repeat with a second layer of cake. Place final cake layer on top, then set in refrigerator 30 minutes to set. Once set, frost cake with remaining icing. Place cake in freezer 10 minutes to set frosting. Once frosting is set, carefully place snowflake stencil on top of cake and dust with powdered sugar. Sprinkle top of cake with remaining sea salt and caramel sauce. Serve immediately!