Preheat your oven to 180°C and line a baking tray with baking paper.
Beat the butter, vanilla and sugar until very pale and fluffy. Beat in the egg, then sieve the flour, baking powder and salt together and fold into the butter.
Gently mix through the chocolate. Roll into balls, place on the tray and gently press them down.
Bake until golden, approximately 13 to 15 minutes.