1. Preheat your oven to 350°F/176°C. Grease line two 4-inch cake pans with parchment paper.
2. Add all-purpose flour, sugar, baking soda, and baking powder into a mixing bowl. Whisk until well combined. Add vinegar, melted butter, vanilla, and milk and mix until well combined.
3. Split the batter equally into both the prepared cake pans. Bake for 20-25 minutes or until a toothpick inserted into the center of the cake comes out clean. Cool the cakes completely on a wire rack.
4. Make the buttercream by adding the butter, powdered sugar, milk, vanilla, and salt into a mixing bowl. Mix with a hand mixer or stand mixer until light, pale, and fluffy.

Assemble:

1. Use a serrated knife to level off both the cooled cakes so they have flat tops. Take one layer and spread ¼ cup of buttercream onto it into a smooth, even layer. Top with the second layer of cake. Pop the cake into the fridge for 15 minutes.
2. Crumb coat the entire cake with a thin layer of buttercream and refrigerate for 15 minutes. Then, frost the entire cake with the leftover buttercream.