• Preheat the oven to 350°F (175°C). Grease an 8x8-inch pan and set aside. Rinse and drain the blueberries, then toss them with 2 tablespoons of the self-rising flour taken from the measured 2 cups so they are lightly coated.

For the Cake:

• In a large bowl, beat the room-temperature butter with the granulated sugar until pale and slightly airy. Add the egg and vanilla and mix until combined.
• Incorporate the self-rising flour gradually, alternating with small additions of milk, until the batter is smooth and light in texture.
• Fold the flour-coated blueberries gently into the batter to avoid crushing them. Transfer the batter to the prepared pan and level the surface.

For the Crumb Topping:

• Combine the light brown sugar, 2 tablespoons granulated sugar, all-purpose flour, and salt in a medium bowl. Cut in the cold, cubed butter with a pastry cutter or fork until the mixture resembles coarse crumbs.
• Evenly sprinkle the crumb mixture over the cake batter in the pan.
• Bake in the preheated oven for 55–60 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Remove from the oven and allow the cake to cool in the pan.

For the Glaze:

• Whisk together the powdered sugar, milk, and vanilla in a small bowl until smooth and pourable.
• Once the cake has cooled completely, drizzle the glaze over the top. Store the cake in a covered container at room temperature for up to 3 days.