• Grease and line two 6-inch cake pans. Preheat the oven to 350°F/180°C.
• Whisk the flour, baking soda, cocoa powder and salt in a small bowl.
• Beat softened butter, oil, and sugar together in a large bowl until light and creamy. Whisk in the egg, vanilla, vinegar, and red food color.
• Add your dry ingredients to the wet ingredients and fold them together, along with the buttermilk.
• Divide the cake batter evenly between prepared pans.
• Bake for around 25 minutes until a toothpick inserted into the middle comes out clean.
• Let the cakes cool in the pan for 5 minutes before inverting onto a wire rack and cooling the cake layers to room temperature.  

Ermine frosting

• While the cakes are baking, make the frosting so the flour mixture has time to cool. If you're making cream cheese frosting, this doesn't need to be started in advance.
• Whisk the flour and granulated sugar in a saucepan to remove any lumps. Whisk in the cold milk.
• Place the mixture over medium heat, stirring constantly until it thickens into a custard-like consistency.Set it aside and let it cool completely.
• With an electric beater or a stand mixer with the whisk attachment, beat the room-temperature butter at medium speed until it's light, fluffy, and lightened in color. It takes around 4-5 minutes.
• Once your butter is fluffy, reduce to low speed and add your cold flour paste to the beaten butter, a spoonful at a time.
• Add the vanilla extract and the pinch of salt. Continue beating on high speed until your frosting is light and airy.

Cream cheese frosting - The other frosting option

• beat the cream cheese and butter with an electric mixer until smooth in a large bowl. Add the powdered sugar, vanilla extract, and salt and continue to beat until the mixture is creamy and smooth.
• If the frosting is too thick, add a teaspoon of milk or heavy cream until the desired consistency is reached. It should be soft but not runny. If it is too thin, add more powdered sugar until it thickens.

Assembling

• Use a serrated knife to level the cooled cakes and make their tops flat and even. Take one cake layer and spoon on ½ of the frosting. Use an offset spatula to smooth it out.
• Top it with the other cake layer. Spoon on the remaining frosting and use the offset spatula to spread it on top and the edges of the cake. You can also add frosting around the sides of a cake and use an icing smoother to smooth it out.
• Crumble up the offcuts of the cake and use them as decorative crumbs on top. Store any leftovers in an airtight container.