Preheat the oven to 180°C. Line a muffin/cupcake tin with 4 paper cupcake cases and set aside.
Melt the butter in a small microwave jug. Allow to cool slightly and then add the sugar and whisk together well. Add the egg and vanilla, and whisk again.
Sift in the flour and baking powder and add the salt. Pour in the milk and combine gently until all ingredients are incorporated.
Evenly distribute the mixture between the 4 prepared cupcake cases and bake for 15-17 minutes until the cupcakes are starting to go golden brown and a toothpick inserted into the centre comes out clean.
Remove from the oven and sit for a couple of minutes before transferring to a cooling rack. Allow to cool completely before frosting.
Combine all ingredients using a spoon or a small hand beater.
Either pipe, or use a knife and spread onto the cooled cupcakes.
Top with sprinkles.
Frosted cupcakes will keep well in an airtight container at room temperature for a few days.