In a pan over medium heat, add the sugar and let it melt.
Next, add in the lemon juice and vanilla extract and turn the heat down.
Once the sugar has melted, add in the blueberries and let them cook for 8-10 minutes or even longer, until the blueberries start break and get mushy.
Sprinkle the cornflour over this, and keep stirring continuously until the mixture thickens, then take it off heat.
Notes
If your sauce doesn't seem to be thickening as much as you want it to even after adding the cornflour, be patient and allow it to cool down completely. As it cools down, it will thicken up even more.
If you're serving the sauce with a warm food like oatmeal or waffles, you may want to refrigerate it first.
You can also add in some other fruits like strawberries, raspberries and even peaches into the mix to make a mixed fruit sauce. Just remember to chop them up so that they're roughly the same size as the blueberries.
This sauce is meant to be slightly chunky and retain the texture of the blueberries. If you don't like it and want it to be smooth, just blitz the mixture using an immersion blender once it has cooled down a bit.