In a small saucepan over medium heat, combine blueberries, granulated sugar, and lemon juice. Cook for about 5 minutes, stirring until blueberries soften. Remove from heat and let cool.

Mix cornstarch with water in a small bowl and stir into the cooled blueberry filling. Cook for 1-2 minutes until thickened, then set aside to cool.

In a mixing bowl, beat softened cream cheese until smooth. Add granulated sugar, vanilla extract, and egg yolk, mixing until creamy. Refrigerate for 15 minutes to firm up.

Place a wonton wrapper flat on a clean surface. Spoon 1 teaspoon of the cheesecake mixture into the center, then top with ½ teaspoon of the blueberry filling.

Brush edges of the wrapper with beaten egg, fold to form a triangle, and press edges tightly to seal. Repeat until all filling is used.

Heat oil in a skillet to 350°F (175°C). Carefully drop wontons in batches, frying for 2-3 minutes per side until golden brown. Use a slotted spoon to remove and drain on paper towels.

Dust cooled wontons with powdered sugar before serving warm.