Preheat oven to 325° with rack in the top third of the oven. Spray Twinkie pan well. Make the simple syrup by mixing the sugar and hot water together until sugar is dissolved. Set aside.
Beat eggs until thick, about a minute. Add oil, and milk or water and mix until well incorporated. Add Vanilla Cake Mix and beat on medium for two minutes (batter thickens slightly and becomes lighter in color). Pour batter into sprayed Twinkie pan or pans only until half full. You do not want batter to flow over edge, it will form a lip (if this happens, trim when cool with sharp scissors).
Bake 13 to 18 minutes. Cakes will spring back when touched and a toothpick will come out clean. Edges will be lightly golden. Do not overcook or cake will be dry. Cool on a rack for 10 minutes or until cool enough to handle. Remove gently from pan, being careful not to tear the cakes.
Using a long straight straw, start at one end of the cake and push the straw horizontally through the center of the cake to carefully carve out a tunnel for the filling. Be careful not to break through the other end. Pull straw out, and blow crumbs into the sink. Repeat this with all the twinkie cakes.
In a tiny bowl mix together salt and hot water until dissolved, and set aside. In a large bowl, combine marshmallow crème, shortening, powdered sugar, and vanilla; beat on high until light and fluffy, scraping down sides as needed. Add salt solution and beat again at medium/high speed for one minute.
Place frosting tip in pastry bag or corner of plastic bag. Fill bag with filling and secure the end. Line a sheet pan with parchment paper. Hold a cake in one hand and place the pastry tip in the other end as far as the tip will go, squeezing filling into the cake. Fill, being careful not to burst the cake. As cake fills pull the tip slowly out while still squeezing. (The bigger the hole in the tip, the easier it is to squeeze the filling.) Once filled, place on sheet pan and move on to the next one, filling all the cakes.
Using a soft brush, paint each Twinkie with simple syrup on the top and all sides. Let sit at least 30 minutes for syrup to soak in before serving.
Store in an airtight container.
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