Preheat oven to 200 degrees C and grease a 23x33cm baking tin.
Sift together the flour, baking powder and salt. Cream the butter and sugar in a mixer until pale, about 3 minutes. Beat in the eggs one at a time then vanilla. Beat 5 minutes.
Add the flour mixture alternately with the milk in about 4 batches. Mix until just smooth, then pour into the baking tin. Bake 20 minutes or until it tests done.