I used to make this when I was ten:

pre-heat oven to 180• celcius.

Combine in a bowl:
5 cups flour (not self raising)
3 cups sugar
1 cup melted butter
4 cups buttermilk
3 tsp baking soda
3 tbsp cocoa powder
2 tbsp ground cinnamon

Distribute evenly in well greased, large tin (12 by 15 inches or so) and bake for about 45 minutes till a cake needle comes out clean.

Will keep for all eternety in the fridge, but tends to disappear rather quickly.

Oh, and it's called Saupkake, literally "buttermilk cake".