Vegan Guinness Chocolate Cake with "Baileys" Buttercream

Name: Vegan Guinness Chocolate Cake with "Baileys" Buttercream
Prep time: 15 min
Cook time: 30 min
Total time: 45 min
Cuisine: European, British
Ingredients: 250 g / 2 cups plain flour; 120 g / 1 cup unsweetened cocoa powder; 2 tsp baking soda; 220 g / 1 cup caster sugar; 100 g / 1/2 cup light brown sugar; 1/2 tsp salt; 480 ml / 2 cups Foreign Extra Guinness, this is apparently the only vegan Guinness; 120 ml / 1/2 cup vegetable oil, I used mild olive oil; 2 tsp white vinegar or apple cider vinegar; 1 tsp vanilla extract; 250 g / 1 cup vegan butter, I used 'Pure' sunflower butter; 600 g / 5 cups icing sugar; 2-3 tbsp Irish Whiskey, I used Jamesons; 1/2 tsp cocoa powder; 1/2 tsp very strongly brewed coffee