Preheat the oven to 350F (180C). Grease and line with parchment paper one 8x9 inch (20x23cm) baking tray.
Beat the butter and sugar together on medium-high speed until combined.
Add the oil, egg and vanilla extract and beat on high for 2 minutes.
Beat in your desired amount of food coloring just until combined. I used 1 teaspoon of red gel food coloring.
Whisk the flour, baking soda, cocoa powder, and salt together in a bowl.
With the mixer on low speed, add the dry ingredients in 2-3 additions alternating with the buttermilk.
Lightly beat the egg white with a fork, then fold it into the cake batter.
Pour the batter into the prepared baking tray and spread evenly.
Bake for about 20 minutes or until a toothpick inserted in the center comes out clean.
While the cake is still hot, carefully trim the edges to neaten.
Take a kitchen towel and spray water on it. Flip the cake over the wet towel and remove the parchment paper. While still hot roll the cake up in the towel. Let it cool to room temperature.