Preheat the oven to 180°C and line 3x 6 inch cake pans with cooking spray and baking paper.
In a small bowl combine white vinegar and dairy-free milk. Set aside. In a large bowl, whisk to combine white sugar, vanilla, and canola oil.
In a separate bowl, sift flour and baking powder and stir to combine. Add half of the dry ingredients into the wet and half of the milk mixture and whisk until combined. Add in the remainder of the dry ingredients along with the milk and again stir until mixture just comes together being careful not to overmix. Finally add in the salted caramel sauce and whisk until combined.
Divide the batter evenly between the 3 pans and place in a preheated oven to bake for 18-20 minutes, or until the cake just springs back when lightly pressed.
Take the cakes out of the oven and let cool in the pan for 10 minutes before turning out onto a wire rack to cool completely.
Make the buttercream. Using a hand mixer or a stand mixer with the paddle attachment fitted, beat dairy-free butter and salted caramel sauce for 1-2 minutes until creamy. Add in 2 cups of icing sugar and beat on medium until combined. As mixture thickens, add inthe dairy-free milk as needed. Add in an additional 2 cups of icing sugar and more dairy-free milk as required until it's smooth and creamy but still thick. If frosting is too thin, add in an extra ½-1 cup of icing sugar until it reaches the right consistency.
Decorate the cake. Place a small amount of the buttercream on a cake stand and place the first cake layer down. Add enough frosting to thickly cover the layer and add the second cake layer. Add more frosting, then the final cake layer upside down so the top of the cake is flat. Add more frosting to the top and side of the cake and use a cake scraper or knife to smooth out the edges and remove any excess frosting. This will be your crumb coat. Place the cake in the fridge for 30-60 mins to chill.
Add another layer of buttercream to the tops and sides. Use your scraper or cake knife to smooth out the sides. Place the cake back in the fridge again to chill. Any excess frosting can be placed in a piping bag with an open star tip (I used a 1M).
To make the caramel drip, you want to play around with the consistency of the carmel. The caramel will continue to drip down the cake more than a ganache, so you want the caramel thicker than you might expect. Leave it in the fridge to thicken up to the right consistency. I like to use a squeeze bottle for more control and carefully place a small amount of caramel at the top of the cake and watch it drip down. Once you get a feel for it, continue to move around the cake. Finally, spoon excess caramel over the top of the cake to coat the top of the cake.
Finish the cake by piping any excess buttercream around the top. Store the cake in the fridge to help the caramel drip set.