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, plain 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, fill almost to the top as this gives them a good rise.
Bake in the oven for 20 minutes until golden brown and a skewer inserted into the centre comes out clean. 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 whole in each cupcake deep enough for a filling being careful to preserve a cake 'lid'.
Remove the filling from the fridge and spoon some filling inside the cake. 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 add butter or blackberry purée if too thick.
Pipe with your preferred nozzle and place a blackberry on top to decorate.