Beat the butter, sugar and vanilla paste, using electric beaters or a freestanding mixer, until pale and fluffy. Add the egg a little at a time, at a slow beating speed until each bit is incorporated fully. Slowly pour in the chocolate making sure to beat all the time. Next slowly beat in half the flour until just combined, then the other half until just combined (Don't over beat). Divide between the tins and bake for 35-45 minutes; they should be well risen but with a slight wobble. Cool completely in the tins.