Beat the butter and cream cheese together on high speed until light and fluffy and smooth.
Sift the powdered sugar and slowly add, one cup at a time, mixing after each addition. Add the sea salt and mix until incorporated.
Add the vanilla bean paste and then the heavy cream, slowly one tablespoon at a time, until incorporated and the mixture reaches spreadable consistency.
Mix the frosting on medium-high speed for about 1-2 minutes, or until the frosting becomes very smooth and spreadable.
Reserve about 1 1/2 cups of frosting for the crumb coat (the initial coat of frosting) and place the rest of the frosting in a piping bag fitted with a star tip. Set aside.
Trim the cakes (if necessary) to level them, using a serrated knife. Place one cake, right side up, on a plate or cake board.
Add a large dollop of frosting on top and spread it out to reach the edges of the cake. Place the other cake on top of the frosting, upside down so the flat side faces up.
Add the remainder of the frosting (the 1 1/2 cup you reserved for the crumb coat) to the top of this cake and spread it over the top and down the sides of the cake, making sure to smooth it out to fill in any holes or cracks in the cake to create a very smooth edge.
Remove any excess frosting - it's fine if the cake is showing through, you'll cover this with the final coat of frosting.
Place the cake in the fridge to chill for about 45 minutes. Refrigerating the cake will cause the butter and cream cheese in the frosting to harden and it will create the perfect canvas for the second coat of frosting.
Remove the cake from the fridge and get ready to frost! If you're not into rosettes you can feel free to simply spread the remaining frosting on if you'd prefer that look.
For the rosettes, pipe a round, spiral shape beginning from the middle and moving to the outside. Follow my friend Liv For Cake's tutorial HERE.
Start piping the rosettes on the side of the cake and begin at the very bottom. Pipe a row of rosettes along the bottom and then offset your second row so it fills in the empty spaces.
On the top of the cake, pipe the rosettes around the outside first and then work your way in, offsetting the rows to fill in the gaps. Make sure you pipe the rosettes close together so they're almost overlapping each other. This will ensure you don't have many empty spaces to fill. It's also a good idea to end your rosettes in the gap where the next row will fill. This will hide the scraggly ends of the rosettes and create a really clean look.
Once you've finished frosting your rosettes, chill the cake for another 1-2 hours before serving to set the frosting. After you pull it out of the fridge it will be quite hard, so it's a good idea to let it sit at room temperature again for about 15 minutes before slicing and serving.