The BEST Chocolate Mud Cake

Name: The BEST Chocolate Mud Cake
Prep time: 30 min
Cook time: 25 min
Total time: 55 min
Cuisine: American
Ingredients: 1 3/4 cup (220g) plain flour; 1 3/4 cup (350g) caster sugar; 3/4 cup (65g) cocoa powder; 1 tsp baking powder; 2 tsps bi-carb soda; 1 tsp salt; 1 cup (250ml) buttermilk (see tips); 1/2 cup (125ml) vegetable oil; 2 large eggs (at room temperature); 1 tsp vanilla extract; 1 cup (250ml) coffee (hot and strong); 290 g unsalted butter (softened to room temperature); 3-4 cups (360-480g) icing sugar; 3-5 tbs (45-75ml) milk; 1/4 tsp salt (optional)