Nigel Slater's Extremely Moist Chocolate Beet cake

There are options, probably more than here, but for a start:
Make 2 cakes in succession, one full recipe each in the 9" pan. This pan is 1.5x vol (6c vs 4c) of the 8" pan recipe recommends, so it won't hold a double recipe. Cake will be lower and wider. Check sooner than original directions say for done-ness.
Look around your kitchen and think of what else you could use to bake in. Loaf pan? Cast Iron Skillet? Muffin pans? I've had great results making cakes or muffins from cake recipe in all these. Adjust your baking and testing time accordingly.
Make only one Beet Chocolate Cake for the 16 people (even though it serves 8) by serving small pieces with more garnishes and/or more desserts. They, their will power and their waistlines may even thank you.