Into a saucepan, add all the butter, dark brown sugar, vanilla extract, kosher salt, and heavy cream, and melt them over medium-high mixing constantly until the sugar and butter melts.

Bring it to a simmer, and simmer for about 3 minutes until the mixture thickens. Remove from heat and set it aside to cool.

Into a large mixing bowl, add the softened butter, dark brown sugar, and kosher salt. Mix with an electric hand mixer until well combined.

Add the eggs, and mix until well combined and the mixture is smooth.

Add the sour cream, oil, vanilla extract, and vinegar. Mix well until combined.

Add the ½ of flour, and add the baking powder and baking soda, add ½ of the buttermilk and mix until almost combined.

Add the rest of the flour and buttermilk. Mix until just combined. Do not overmix this.

Divide the batter into 3 8-inch baking pans and bake in a 180c (350f) preheated oven for about 25-30 minutes (depending on your oven) or until a toothpick is inserted into the cake and comes out clean.

Let the cakes cool to room temperature and remove them from the pans.

Into a big mixing bowl, add the room temperature butter, salt, powder sugar, vanilla extract, cooled butterscotch, and milk. Mix with an electric hand mixer for a few minutes until the buttercream is light in color and creamy.

Pipe a small dollop of the buttercream on the surface that you will frost on. Lay the first cake layer over the frosting dot.

Top the first cake layer with ⅓ of the buttercream and spread an even layer of the buttercream with a spatula.

Drizzle about 2 tbsp of the butterscotch sauce and spread it with a spoon.

Top the cake with the second cake layer. Repeat adding the buttercream and butterscotch sauce and top with the third cake layer.

Frost the cake with the remaining buttercream and pour the rest of the butterscotch sauce over the cake. Smooth it out with a spatula, creating a marble effect.