Preheat oven to 160C (315F/Gas Mark 2-3).
Line a muffin tray with cupcake cases.
Beat margarine and caster sugar together until light and fluffy (approximately 5 minutes with an electric whisk or stand mixer).
Add eggs, 1 at a time, beating well after each egg until fully combined.
Add vanilla extract and mix to incorporate.
Add self-raising flower and bicarbonate of soda, mix on a very slow speed (or fold in with a spatula), until just incorporated.
Add milk and mix in gently, to slightly loosen the mixture.
Divide the cupcake batter equally between the 12 cupcake cases.
Bake on the middle shelf of the oven for 20 minutes, until golden and springy to the touch.
Remove from oven and allow to cool for 5 minutes, before removing from muffin tray to cool completely on a wire cooling rack.