Adapted very slightly from The Vanilla Bean Blog (but really from Tessa Huff’s Layered).
Note that the cake pictured here was made in 6.5" cake pans, while this recipe is meant for 8" pans. If you want to make a 6.5" cake, you can half the (whole) recipe and bake the cakes in two 6.5" pans for 30-35 minutes.
You can adjust the amounts of rum in the rum-espresso soak and the buttercream depending on how much you like rum. I'd caution against increasing it too much though, especially in the buttercream; you don't want the buttercream to get too loose.