Preheat the oven to 180C/350F and spray a 2lb loaf tin with cooking spray. Place a strip of greaseproof paper across the loaf tin with enough overhang to lift the cake out later.
Place the butter, sugar and orange zest into a large bowl, or the bowl of your stand mixer and beat until light and fluffy. This will take around 2 minutes on medium-high speed.
Add in the eggs, scraping down the sides as necessary, and mix until well incorporated, around 1 minute on medium speed.
Place the flour, baking powder, salt, cinnamon and chocolate chips into a medium sized bowl and stir until the chocolate chips are coated in flour.
Tip the flour mixture into your egg mixture and mix on low until the mixture starts to come together, then add the milk gradually while continuing to mix. Once all the milk is added, turn the speed up to medium and mix until everything is smooth and well incorporated.
Spoon the mixture into the prepared loaf tin and smooth into an even layer. Place in the oven for 60-70 minutes, until risen, golden and an inserted skewer into the centre comes out with no cake crumbs. You will get melted chocolate on the skewer, just make sure there's no cake batter on it.
Leave to cool in the pan completely, before lifting out and transferring to a wire rack with kitchen paper underneath it.
Make the glaze: Sift the icing sugar into a small bowl, add in the cinnamon and whisk briefly to disperse. Add in milk, 1/2tbsp at a time, whisking until you have a thick, smooth mixture which ribbons when you pull the whisk out. I used 1tbsp in total.
Drizzle/spoon the glaze over the top of the loaf cake. You can then sprinkle fudge pieces or sprinkles on top if desired. You can skip the glaze entirely if you'd rather and just serve the cake as is.
Chocolate Orange and Cinnamon Loaf Cake will keep in an airtight container, at room temperature, for 4 days. It will store better without the glaze. You can enjoy this cake at room temperature, or heat a slice in the microwave and serve with a lump of melting butter.