Preheat oven to 180c. Grease your bundt pan really well with a teaspoon of butter. In a mixer bowl beat the eggs, add in sugar and beat for a minute. Add in oil and vanilla essence and beat until combined. In a measuring jug (or bowl), combine flour, baking powder and salt, add half this mixture into the bowl and beat for a few seconds add the buttermilk and the last half of the flour mixture. Beat until just combined and pour into the pan. Place in the oven for 25-30 minutes or until a skewer comes out clean.
While baking make the glaze. Place a small pot onto the stove at a medium heat. Throw in butter and sugar and stir until sugar has dissolved, add in cinnamon and icing sugar. At this stage the mixture will be thick, mix as best as you can and then add milk to reach the desired consistency, set aside.
Once baked let cool and turn onto a serving plate (you may have to cut a little of the top if it domes while baking). Drizzle with the glaze and serve.