To make the dough: In a large mixing bowl, place the flour, cornflour, sugar, yeast and salt. Mix to combine.
In a small pan, gently heat the milk and butter until the butter has melted and the milk is just warm.
Add the wet ingredients to the dry and mix until a dough comes together. Knead the dough by hand or with a dough hook attachment in a stand mixer for 7 minutes until smooth and elasticated. Place the dough in an oiled bowl, cover and leave the dough somewhere warm for 60 minutes or until the dough has doubled in size.
To make the filling: Cream together the sugar, butter and cinnamon until thoroughly combined. Set aside for later.
Grease the base and sides of a 23cm x 30cm / 13 x 9-inch baking tin with extra butter. When the dough has doubled in size, tip it out the bowl and knock the air out. The dough is very soft, so you might need to add some extra flour to make rolling out easier. Lightly flour a work surface and roll the dough into a large, neat rectangle about 50cm x 30 cm / 20 x 12-inches and with one of the longer sides nearest to you.
Evenly spread the cinnamon mixture over the dough, ensuring the mixture reaches all the edges.
Starting with the long side of the rectangle, roll the dough up into a sausage shape. Using a sharp knife (I find a serrated bread knife best for this), trim the edges of the dough and then cut the dough into 12 rolls.
Place the rolls side by side in the baking tin, with the spiral facing upwards. Cover and leave somewhere warm until they’ve all joined up and have nearly doubled in size – this should take approx. 45-60 minutes.
Preheat oven to 200°C / 180°C Fan / 400°F / Gas Mark 6. Bake the rolls for 10 minutes, then turn the oven down to 180°C / 160°C Fan / 350°F / Gas Mark 4 and bake for a further 15-20 minutes until the rolls are a light golden colour on top. Leave to cool in the tin for about 15-20 minutes.
To make the icing: Mix together the icing sugar and enough water until the icing reaches your preferred consistency. Drizzle the icing over the top of the cinnamon rolls and leave to set for about 5-10 minutes.
Once the icing has set, the cinnamon rolls are ready to eat. The rolls will keep in an airtight container at room temperature for up to 2-3 days.