For the cake: Preheat the oven to 180C/Fan 160C/Gas Mark 4. Then grease and line a 33cmx22cm cake tin and set aside. In a large mixing bowl, cream the butter and sugar together using an electric whisk light and fluffy, Then add the vanilla and eggs, and beat again until just combined. The mixture will look a little curdled at this point. Then add in the flour, baking powder and salt and mix until just combined. Finally, add the milk again mixing until just combined. Set aside.
For the cinnamon swirl: Melt the butter and pour into a small bowl. Add the sugar, flour and cinnamon and mix to combine until a thick paste forms.
Pour 2/3 of the cake batter into the prepared cake tin and spread evenly. Then scatter the cinnamon swirl paste evenly over the cake batter. Then add the final 1/3 cake batter and spread gently over the top of the cinnamon swirl paste.
For the topping: In a small bowl, mix together the cinnamon and sugar. Then sprinkle evenly over the cake batter, then place into the oven to bake for 25 minutes.
Take the cake out of the oven and place on a cooling rack to cool for 5 minutes. Then remove the cake from the tin, and place back onto the cooling rack to cool completely.
Cake can be kept in air-tight container for 2-3 days.