With a hand held electric mixer, cream together the sugar and butter until pale in colour and fluffy. Add the eggs and vanilla extract and mix well. Sieve together the flour, baking powder, baking soda and salt. Add this to the creamed mixture and beat till just combined. Do not overwork the dough. If the dough feels sticky, add a little more flour. If the dough seems dry, add a tsp or so of milk to get the right consistency.