Roll the pastry out thinly on a lightly floured surface, then stamp out 24 × 6 cm (2½ inch) circles with a fluted biscuit cutter and press into the buttered sections of 2 × 12-section mini muffin tins, reserving any trimmings. Prick the base of each tart 2–3 times with a fork, then chill for 15 minutes.
Meanwhile, mix the cornflour and measurement water to a paste in a saucepan, then add the sugar and half the blueberries. Cook over a medium heat for 2–3 minutes until the blueberries soften and the juices begin to run. Take off the heat and add the remaining blueberries. Leave to cool.
Roll out the remaining pastry trimmings and cut out 24 tiny heart shapes. Place the heart shapes on a baking sheet, brush with milk and sprinkle with caster sugar.
Line the tarts with small squares of nonstick baking paper and baking beans and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 5 minutes. Remove the paper and beans from the tarts and cook for a further 4–5 minutes until the bases are crisp, cooking the heart shapes for 4–5 minutes on the shelf below.
Transfer the tart cases to a wire rack to cool. When ready to serve, spoon in the blueberry compote and top with the heart shapes.