If you don’t find or want to use buttermilk, you can make some acid milk by yourself: 125 gr dairy free yogurt + 135 ml nut milk + 1 tbsp of lemon juice. Mix all well.
Beetroot powder gives a nice dark red color, nearly purple. If you want to have a nice red use a few drops of food coloring.
You can give the cake batter any form you like. Square or round doesn’t matter. Put the batter thin enough to have 4 equal slices in the end. Or bake already the size and shape you want the cake and just cut the sponge cake horizontally in 4.
You can decorate the cake with all kinds of red fruit or other. A nice decoration with beetroot powder is also a good idea.