In a small bowl, add your milk and vinegar and stir them together. Set aside, and let them set for at least 5 minutes for the milk to "curdle" and become buttermilk.
Preheat your oven to 350 and line a muffin tin with cupcake liners.
In a small mixing bowl, add your gluten-free flour, cinnamon, baking powder and salt.
In a large mixing bowl, add your coconut oil and sugar and whisk them together. Add your eggs, "buttermilk" and vanilla to the bowl and whisk them in too.
Pour your dry ingredients into your wet ingredients and stir them together, to form your cupcake batter.
Take your cupcake batter and fill each cupcake tin about 2/3 way full with cupcake batter, splitting it evenly among all 12 cupcakes.
Bake your cupcakes in the oven for 18-22 minutes, or till the tops are springy to touch and a toothpick inserted comes out clean.
Let your cupcakes cool entirely before topping with frosting.
In a large mixing bowl, add your butter, water and vanilla and beat them together, using a hand mixer, till smooth. Add your cinnamon and one cup of powdered sugar to the bowl, and use a hand mixer and beat them together till smooth. Add your next cup of powdered sugar and beat them together again till smooth.
Pipe or spread your frosting on top of your cooled cupcakes.