Preheat oven to 180°c. Line 2 standard muffin pans with 18 patty pans and set aside.
Sift the flour, baking powder, bi-carb soda and salt into a mixing bowl and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar together until light and fluffy (about 4-5 minutes). Mix in the eggs, one at a time, then the vanilla paste.
Mix in the dry ingredients in three additions, alternating with the buttermilk making sure to mix well after each addition. Use a rubber spatula or spoon to turn the batter and make sure everything is well combined.
Toss the blueberries with the teaspoon of flour and gently fold them into the batter.
Fill the patty pans about ¾ of the way full. For a little crunch, sprinkle each cupcake with demerara sugar then bake for 17-20 minutes or until a toothpick inserted into the centre of the cupcake comes out clean.
Remove from the oven and allow to cool.
For the vanilla custard buttercream
In a small saucepan, stir together the custard powder and a little milk to form a paste. Add the sugar and the rest of the milk and whisk well until smooth.
Heat the mixture over low heat, stirring constantly to prevent lumps and sticking until the mixture thickens so that it drops off the spoon in dollops. It should take around 5 minutes. Remove the saucepan from the heat and cover it with plastic wrap, pressing it down onto the surface of the custard to stop a skin forming. Allow to cool completely: you can put it in the refrigerator to speed up the process.
Once the custard is cool, place the butter and the icing sugar in the bowl of a stand mixer fitted with the paddle attachment and beat well until very light and creamy. Add the cream and mix to combine.
Add the cold custard and beat until the buttercream is smooth and has a good consistency.
Pipe the vanilla custard buttercream onto the cupcakes and enjoy!
Coating the blueberries in flour takes only a few minutes, but it stops them from sinking to the bottom. Genius!Custard powder is not available in all countries. In Germany, custard is either runny, like a vanilla sauce, or designed to put in a mould. If you can't find any custard powder, you can make custard any way you know how. Normally cornflour, sugar, vanilla and milk. My grandma used to use milk powder in hers when she was making it from scratch.If your custard goes lumpy, hit it with a hand or stand blender.