Pre-heat the oven to 350F and prepare a 9x9 inch cake pan with cooking spray and baking parchment.
In a stand mixer fitted with the paddle attachment, beat together the butter with half the sugar until combined. Then do the same with the remaining sugar.
Add the vanilla then beat in the eggs, one at a time until the mixture is smooth, followed by the oil, baking powder, baking soda and salt.
Add in a cup of flour and mix until the mixture is smooth. Then add 1/2 cup of the buttermilk, followed by the remaining cup of flour, then the remaining 1/2 cup of buttermilk.
Combine all the ingredients for the cinnamon layer in a small bowl.
Spread half the cake batter across the bottom of the prepared cake pan in a single layer. Then sprinkle the cinnamon sugar mixture over the top as evenly as you can, making sure it gets into all the edges and corners. Then, dollop the remaining cake batter over the top, gently spreading it out with a spatula so it reaches all the edges and is smooth on top, doing your best not to disturb the cinnamon layer.
Bake for 35-40 minutes until a toothpick or the tip of a sharp knife inserted into the middle comes away clean. Leave to cool in the tin while you make the glaze.
For the glaze, stir 1 tbsp of the milk, the honey, vanilla and salt into the powdered sugar. Then gradually add the final 1 tbsp of milk until you have a relatively runny glaze.
Pour the glaze carefully over the top of the cake, making sure it is spread evenly across the middle.
Allow the glaze to set while the cake cools completely before removing it from the tin and slicing.