Dice 1 cup of strawberries into small chunks and set aside.
Using a blender or immersion blender, puree remaining 1 ¼ cups of strawberries until smooth.
Add strawberry puree, sugar, and ½ cup of diced berries to a saucepan.
Heat contents of saucepan until it begins to boil. Boil for approximately 1 minute.
Remove from heat and let cool. We chilled the mixture in a bowl in the fridge, but you can leave it to cool in the pan or on the counter if you prefer.
Add half of the strawberry mixture to each glass, swirling as you pour to coat the glass.
Add remaining diced strawberries to the glass.
Carefully pour in milk and swirl around gently. This should create the swirled pattern you see in the pictures.