Step 1: Preheat your oven to 350°F (175°C) and grease a 9-inch baking dish. In a small bowl, toss the blueberries with 1/4 cup of sugar and set aside. In a separate bowl, whisk the flour, baking powder, and salt together.
Step 2: In another large bowl, cream together the butter and remaining sugar until light and fluffy. Beat in the egg and vanilla extract until smooth.
Step 3: Slowly add the dry ingredients to the butter mixture, alternating with milk, beginning and ending with the dry ingredients.
Step 4: Pour the batter into the prepared baking dish and spread it evenly. Scatter the blueberries and sugar mixture on top of the batter.
Step 5: To make the glaze, mix cornstarch and water in a saucepan over medium heat. Stir constantly until the mixture thickens into a smooth glaze. Pour the glaze over the cake, spreading it evenly.
Step 6: Bake the cake for 35-40 minutes, or until the top is golden and a toothpick inserted comes out clean. Allow it to cool for 10 minutes before serving.