Place the strawberries and about half a cup of water into a blender and blend until completely smooth. Put a fine sieve over a medium pot and strain the strawberry mixture into the pot. Pour in the sugar and the remaining water (2.5 cups) and mix until well combined. Simmer for about 10 to 15 minutes over low heat to help cook the strawberries and allow the sugar to completely dissolved. Add a pinch of baking soda and stir quickly until all of the bubbles have subsided. Mix in the vanilla extract and cook for 1 minute. Then pour/stir in the milk and mix until well combined. At this time taste your atole and if desired add more sugar, this recipe isn't too sweet. In a small bowl combine the cornflour and water until you have a smooth mixture with no lumps. Stir it into the atole and mix until well combined. Continue to simmer over medium-low heat (while you stir) until the atole has thickened up similarly to gravy consistency. Serve straight away and decorate with strawberry slices if desired. Enjoy!