Line a large baking sheet with baking paper. Spoon teaspoonfuls of yogurt onto the lined tray, spreading the yogurt slightly with the back of the spoon to make a neat round shape measuring about 4cm in diameter. You should get about 10 buttons
Top each yogurt round with a slice of strawberry, if using, then put the baking sheet in the freezer for 1 hour until the yogurt buttons are firm. Peel the buttons off the paper and serve immediately.