- Make the blueberry jam first: Place blueberries, sugar, water and Certo into a deep saucepan (the deeper the better as when it's on a rolling boil it will rise!) 
- Place over a medium heat and stir to coat all the berries in the sugar. Bring to a rolling boil and continue to cook for 8-10 minutes, stirring occasionally so the mixture doesn't stick to the bottom of the pan. 
- After 8-10 minutes, the liquid should have reduced, the blueberries will have burst and be mushy and the mixture should coat the back of a metal spoon. 
- Leave to cool for 15 minutes before making the ice cream base: place the double/heavy cream into a large bowl, or the bowl of your stand mixer and beat on medium-high speed until you have stiff peaks. You want to pull your whisk up out of the mixture and have it hold its peak without the tip bending too much. 
- Add in the condensed milk and vanilla extract and fold in by hand until everything is well combined. 
- Take a 2lb loaf tin and spoon a third of the ice cream mixture into the tin. Spoon a third of the blueberry jam on top and use a skewer to swirl the two together. You want streaks of purple, but not the entire ice cream to be purple. 
- Add another third of the ice cream base gently, then repeat with a third of the blueberry jam. Repeat this one more time until you've used all of the ice cream - you may have 1tbsp or so left of the jam, it depends how much you want to use! 
- Place in the freezer for at least 6 hours, overnight if possible. 
- Serve in bowls with syrup, or in cones with sprinkles and a flake - or just alone! This creamy, sweet ice cream with bursts of intense blueberry jam is perfect alone too! 
- Blueberry Swirl Ice Cream will keep in the freezer for one month.