Preheat the oven to 180 C / Gas 4. Place the cupcake cases in a muffin tin.
In a bowl combine the baking powder, caster sugar, flour and salt and mix together.
Add the eggs, oil, milk and vanilla extract to the mixture and mix well.
Place the mixture into the cupcake cases and fill almost to the top.
Bake in the preheated oven until golden brown and a skewer inserted into the centre comes out clean, about 20 minutes. Remove from the oven and cool.
For the filling:
Add the sugar to the blackberries then add the cornflour and mash your mixture until it is mushy. Set aside in the fridge to chill.
Using a knife cut a hole in each cupcake deep enough for a filling. Make sure to preserve the 'lid' of each cupcake.
Remove the filling from the fridge and spoon some filling inside each cupcake. Place the lid back on.
For the icing:
In a bowl, combine the butter, icing sugar and purée and mix together until it forms a thick consistency and you are able to pipe. Add extra icing sugar if too runny, or add butter or blackberry purée if too thick.
Place the icing in a piping bag and pipe with your preferred nozzle onto the cupcakes. Place a blackberry on top of to decorate.