Preheat the oven to 400f / 200c. Grease the pan. The recipe calls for a 12 x 8 inch (30cm x 20cm) pan but I used an 8 x 8 inch (20cm x 20cm) square pan, and turned the oven down to 375f / 190c as the smaller pan created a thicker cake that I did not want to go too brown on top.
In a large bowl, sift together the flour and baking powder, and add the sugar and nutmeg. In a separate medium bowl, whisk the eggs until fluffy. Add the buttermilk, butter, lemon zest, and lemon juice and mix well until incorporated. Add the wet ingredients to the dry ingredients and mix until just combined. You know the mixing rules --
Spread the mixture into the pan and tumble the blueberries over the top and sprinkle with the demerara sugar. I used less than what was called for in the recipe and I did regret it a bit as it would've given a nice crunchy dimension to the texture, so next time I would use the full whack. Bake for 25 minutes until the top is just golden and a skewer comes out clean. I let mine bake a little bit longer, taking about 30 minutes in my oven.
Makes 12 - 15 regular sized square, or 9 "i didn't know my mouth could open this wide" squares.