The coffee isn’t there to add a coffee flavour, it’s there to boost the richness of the chocolate and the hot water helps to bloom the cocoa powder. Oil is liquid at room temperature so it keeps the cake soft. Using butter is fine, but it solidifies at room temperature so your cake will be firmer. In an ideal world, we’d use avocado oil but I’m not made of money so vegetable oil it is. Any chocolate works for ganache but the ratio is important; if you swap out too much dark chocolate for milk chocolate, the ganache won’t set as firm. You can get around it by using a little less cream if you’re desperate to use less dark chocolate. Get fancy with the ganache if you like. Add some extract. Vanilla.. coffee… orange if you’re feeling really crazy. Now I’m all for a heavy choccy cake, pure chocolate. But sometimes it’s nice to break up the richness with some fruit, especially when strawberries are in season. I personally think the best way to enjoy this is warmed in the microwave for 10-15 seconds per slice with a huge scoop of vanilla ice cream. But you do you.