In a medium saucepan melt the butter over a medium heat and then stir in the flour.
Cook for 1 minute over a medium heat, then add a little of the milk.
Stir vigorously to combine, and when the milk is incorporated, add a little more milk.
When you have added half the milk, use a balloon whisk to whisk in the rest of the milk.
Turn the heat down to low and let the sauce cook gently for 5 minutes, stirring from time to time to prevent it from catching on the bottom of the saucepan. This is to take the raw taste out of the flour.
Take the sauce off the heat and add 150g (5 oz) grated cheddar cheese. Season with black pepper.
Using a stick blender, blend the sweetcorn to a smooth puree. It can take a few minutes to get it silky smooth. (This is optional - if you prefer whole corn kernels in your pasta bake and are not making it for baby led weaning you can leave out this step.)
Fold the sweetcorn puree / corn kernels into the cheese sauce. If the sauce is too thick then add 1-2 tablespoons of the pasta cooking water.
Drain the pasta and pour the cheese and corn sauce over and mix well to combine.
Pour into a 2-2.5 litre / 8-10 cup capacity oven proof baking dish.
Sprinkle over the remaining 50g (1.5 oz) grated cheese and place in the oven. Bake for 15-20 minutes until golden brown.
It will thicken as it cools. I don't recommend freezing this mac and cheese.