In a medium saucepan place butter, water, oil, and cocoa. Bring to a boil or until the butter has melted.
Meanwhile, in a large bowl whisk together flour, cinnamon, sugar, and salt in a mixing bowl. Pour the chocolate mixture into the dry ingredients and mix well. (See Note)
Add the buttermilk and baking soda.
In a small bowl, mix together the eggs and vanilla. Add to the chocolate batter. Pour batter into prepared bundt pan.
Bake for 25 - 30 minutes. Allow cake to cool completely in the pan.
Turn over to a serving plate. Glaze as you please or dust with sugar
Notes
Make sure once that you're quick in beating the chocolate mixture with the dry ingredients because the chocolate mixture would be hot and the flour would start cooking if you let it stand.
Recipe by Baker Street at https://bakerstreet.tv/2011/11/chocolate-cinnamon-bundt-cake/