Preheat the oven to 160C (fan). Butter a 23cm cake tin and line the base with parchment paper.
In a medium mixing bowl whisk together the flour and baking powder. Add the zest of the limes and set aside. Measure out 1 Tbsp of the sugar and set aside for sprinkling over cake.
In the bowl of a free standing electric mixer, fitted with the paddle attachment beat together the remaining sugar with the softened butter until really pale and fluffy. Now gradually add the egg (approximately a tablespoon at a time), beating in between each addition and scraping down the side of the bowl.(*see notes).
Add a 1/3 of the flour and mix just until combined then mix in 1/2 the buttermilk mixing just until combined, repeat process making sure your final addition is flour. Now carefully fold all but a small handful of blueberries into the cake batter.
Pour the cake batter into the prepared tin, spreading an even layer, then sprinkle the remaining berries and sugar on the top, pressing the fruit into the batter a little way.
Bake in the pre-heated oven for 40- 45 minutes or until a cocktail stick inserted into the centre comes out clean.
Run a knife around the edge of cake tin to loosen and then turn out. Let the cake cool for at least 20 minutes on a wire rack before slicing and serving. Zest a little extra lime on the top of the cake. (optional)
*By adding the egg a little at a time and beating in between each addition will result in a really lovely light cake, so it is worth taking your time with this part.