Preheat the oven to 180C/350F/gas mark 4 and place a 10 inch/25cm cast iron skillet in the oven to heat up.
In a large bowl, whisk together the flour, cornmeal, caster and brown sugar, baking powder, bicarbonate of soda and salt.
In a jug, whisk together the lemon zest, melted butter, buttermilk, eggs and vanilla extract.
Add the wet ingredients to the dry and mix to combine.
Carefully remove the hot skillet from the oven. Add the 1 Tbsp of butter and swirl the pan until it is melted and the pan is well coated.
Spoon in the batter and spread in an even layer. Scatter over the blueberries then sprinkle over the Tbsp of granulated sugar. Bake for about 30 minutes, until a skewer inserted into the centre comes out clean. Allow to cool in the skillet for about 20-30 minutes before serving.
The blueberries do have a tendency to sink to the bottom, so if you want to keep some visible on top of the cake like mine then you will need to reserve a handful of them and carefully scatter them over the top of the cake partway through the baking time - about 20 minutes.
If you don't have a 10 inch skillet then you can use a 10 inch cake tin instead (or a deep 9 inch one). There is no need to preheat the tin, simply grease it and line the bottom with baking parchment.