Make the frosting while the brownie cake is cooling. Cream the butter and Crisco until smooth, then add the marshmallow creme. Mix in the powdered sugar, salt and vanilla on low speed until combined, and then cream on high speed until smooth and creamy.
Once the cake is cool, carefully flip it out of the pan and onto a cooling rack. The cake will be upside down. Flip it again, this time onto a cake board or serving plate.
Decorate as desired. To get this look, dye about 1/4 of the frosting pink and scoop into the center of the cake. Use an offset spatula to spread it almost to the edge.
Use an offset spatula and a turntable to create a spiral pattern. Lightly press the flat tip of the spatula gently in the center of the cake and turn the turntable as you lightly drag the spatula in a spiral pattern, all the way to the outer edges of the cake. Scrape off any frosting that overflows onto the edges of the cake.
Scoop the white frosting into a piping bag fitted with Wilton tip 1M. Pipe rosettes around the border, and then another layer to fill in the gap, so that the rosettes overlap.
Toast mini marshmallows using a culinary torch or the broiler in your oven and scatter them around the border.
Finally, add sprinkles to the spiral and the border.
Store in an airtight container and enjoy!