Preheat the oven to 350ºF.  Brush vegetable oil on the bottom on the sides of a half sheet pan (18”x13”)*.  Line with parchment paper, then brush vegetable oil on the parchment paper.  Sprinkle flour over the pan, then tap out the excess, coating the oil with the flour.

Whisk the flour, baking powder, baking soda and salt in a large bowl.  In another bowl, whisk together the 1/2 cup of oil, sugar, milk, eggs and vanilla.  Add the wet ingredients to the dry ingredients and whisk just until the mixture is combined.  Pour half of the batter into another bowl (about 1 1/2 cups of batter).  Add the red food coloring to one bowl and the blue to the other.  Stir just until combined.

Spread the red and blue batter in the pan in alternating stripes, 3 of each color.  (I transferred the batter to ziplock bags and then cut off the tip.  I think this made less of a mess for me, but you can also pour the batter!)  Each stripe will be about 2 inches wide. Don’t worry if they aren’t completely straight.

Bake in the preheated oven until the top springs back when pressed, about 15 minutes.  Let the cake sit for 3 minutes, then loosen the edges  from the pan with a knife, if needed.  Generously dust the top of the cake with powdered sugar, then place a kitchen towel over the top. DO NOT use a terry cloth towel.  Place a cooling rack on top, then invert the cake over onto the towel and rack.  Remove the parchment paper, then roll the cake up with the towel, rolling the long end of the cake.  Let the cake cool completely, at least 1 1/2 hours.

To make the frosting, beat the butter and the salt in a stand mixer until light and creamy, about 1 minute.  Add the marshmallow cream and vanilla and continue to beat for 2-3 minutes longer.  Add in the powdered sugar, then start adding the milk, 1 tablespoon at a time, until you reach a spreadable consistency.

Gently unroll the cake and spread half of the frosting over the entire surface of the cake, all the way to the edges.  Re-roll the cake (without the towel).  Trim the edges.  Using the remaining frosting, frost the outside of the cake.