Preheat oven to 325°F and generously grease a 9-inch Bundt pan.
In a bowl, sift together the flour, baking powder, and salt.
In a separate bowl, cream together 6 tablespoons of butter and vanilla. Gradually add the granulated sugar and oil, mixing until well combined.
Beat in the three whole eggs and 3 yolks until smooth.
Alternate adding the flour mixture and buttermilk to the wet ingredients, mixing until the batter is smooth.
Pour the batter into the Bundt pan and bake for 60-70 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for 20 minutes before inverting it onto a wire rack. Let it cool completely for an additional hour.
In a mixing bowl, combine marshmallow crème, ½ cup of softened butter, ½ cup of powdered sugar, and vanilla, mixing until smooth.
Use a paring knife or cupcake corer to make holes in the cake.
Transfer the marshmallow crème to a piping bag or plastic bag and fill the holes in the cake with the cream. Replace the small cake circles back into the cake.
Carefully place the cake onto a serving platter, dust with powdered sugar, and enjoy!