Preheat an oven to 350 F. Combine the ingredients for the butter-flour paste. Then use a silicone pastry brush to spread the mixture inside a 10-inch bundt pan.
In a stand mixer, cream the butter and both sugars for 5-7 minutes until the mixture is light and fluffy. Scraping the bowl down occasionally.
Add in the mashed sweet potato and vanilla extract. Mix until combined.
Add in the eggs one at a time until each one is incorporated.
Combine the ingredients from the flour to the sea salt. Then add the flour mixture to the bowl and mix slowly until just incorporated.
Place the batter into the prepared bundt pan. Spread the batter smooth with an offset spatula.
Bake the cake for 50-53 minutes or until the cake is set.
Allow the bundt cake to cool until it is cool enough to handle with your hands. Using a #809 Wilton piping tip, poke holes and remove cake from 8-10 spots evenly spaced around the bottom of the bundt cake.
Fit the #809 piping tip into a large piping bag and fill it with the marshmallow creme. Generously pipe the marshmallow creme into each hole. Flip the cake upside down onto a board. Give it a tap to release it from the pan.
Prepare the glaze ingredients by whipping the butter and both sugars until creamed. Add in the milk and vanilla extract. Continue beating on high speed until creamy and smooth. Drizzle the glaze over the cake.
Tips
The fastest way to make mashed sweet potatoes is in the microwave. Prick a couple of large sweet potatoes 3-4 times with a fork. Place them on a plate and cook them on high for 5 minutes, flipping halfway through. Repeat this process if the sweet potatoes are still hard in spots, checking them frequently and turning them each time until softened. Allow them to cool. Then remove the skin and mash the potato to further cool before using it in the cake batter. Alternatively, you can roast them in the oven for an even better flavor. Avoid boiling sweet potatoes as it picks up too much moisture for the cake.
To help fill the piping bag easier, fold the piping bag over a tall glass. Fill the bag with the marshmallow creme and then fold the piping bag over the creme.
