Microwave steamable diced sweet potato as directed. Then, mash sweet potatoes until smooth.
Prepare box cake mix as directed (with eggs, oil and water). Add sweet potato mash and stir to combine.
Melt 2 Tablespoons butter. Stir in 2 Tablespoons brown sugar.
Lightly grease and flour two 7” springform pans (or spray with a nonstick flour baking spray.)
Pour ½ of batter into each pan. Divide brown sugar butter mixture each pan and dollop onto the batter in each pan. Gently swirl with a butter knife.
Place into the oven for about 40-45 minutes (or until a toothpick comes out clean.)
Remove cake from the oven and let it cool for 15 minutes.
While the cake is cooling, prepare frosting. Add marshmallow fluff and softened cream cheese to a bowl and mix on high speed until smooth.
In a saucepan, bring 1/4 cup butter and 1/2 cup brown sugar to a boil. Lower heat to simmer for 5 minutes. Remove from heat and stir in pecans.
Remove one of the cakes from the pan and place on a cake stand. Spread all the frosting in an equal layer on top of the cake. Place the other cake layer on top of the frosting and press down gently.
With an offset spatula, spread out any frosting on the sides that ooze out.
Pour candied pecans on top of cake and top with marshmallows (optional).
Notes
I am using springform pans for this cake to make it easier to release after baking. They are 7-inch springform pans so the cake will be thicker. You can certainly use 8-inch round baking pans. They will just make a thinner cake. Reduce baking time by about 8-10 minutes (just use the timing on the back of your cake mix box.)
If you have leftover cooked sweet potatoes, you can use those for this recipe instead of the bagged steam-able sweet potatoes.
This can be made in a 9"x 13" baking dish. Just follow baking times on the back of your boxed cake mix.