Preheat oven to 175 degrees C. Grease six 10cm ramekins and place in a 22xm x 32cm baking pan.
Mix blueberries, lemon zest and 2 tablespoons white sugar together in a small bowl.
Microwave 30g butter in a microwave-safe bowl until melted, about 30 seconds. Stir in brown sugar. Spread equal amounts into the bottom of each ramekin. Divide the blueberry mixture evenly into the ramekins.
Beat 125g butter and 1/2 cup caster sugar in a large bowl using an electric mixer until fluffy. Add eggs 1 at a time, mixing after each addition. Mix in lemon juice and vanilla extract. Add flour, baking powder, ginger and salt; stir until combined. Mix in milk. Spoon batter into the ramekins over the blueberry mixture.
Bake in the preheated oven until a toothpick inserted into the centre comes out clean, 25 to 30 minutes.
Let cool in the ramekins for 5 minutes. Run a knife around the edges of the cakes; invert onto a plate.
If the batter looks too dry, add in a little milk at a time until the desired consistency is reached.