Whisk together egg yolks, lemon juice, milk, sugar, cornstarch, and lemon zest in a saucepan. Cook over medium heat, stirring constantly, until the mixture thickens to a custard-like consistency. Remove from heat and refrigerate until cool.
In a large bowl, combine flour, sugar, salt, baking powder, and lemon zest. In a separate bowl, whisk together melted butter, egg, milk, yogurt, and lemon juice. Gently fold the wet ingredients into the dry ingredients until just combined. Be careful not to overmix. Gently fold in blueberries.
Fill muffin liners 3/4 full with muffin batter. Using a spoon, add a tablespoon of lemon cream into the center of each muffin.
Preheat the oven to 200°C (390°F) and bake for 25 minutes, or until the tops are golden and a toothpick inserted into the edge (not the cream center) comes out clean.
In a small saucepan, combine sugar and water. Heat until it reaches 115°C (240°F) on a candy thermometer. Meanwhile, beat egg whites until soft peaks form. Slowly add the sugar syrup to the egg whites while continuing to beat until stiff peaks form.
Transfer the meringue to a piping bag and pipe onto the cooled muffins. Lightly toast the meringue with a kitchen torch or briefly broil in the oven until golden brown.