Put the butter and sugar in a bowl and cream everything together in a stand mixer until it is light and fluffy. You could also use a hand-held electric whisk or wooden spoon for this. Add two of the eggs and half of the flour and beat well, then add the remaining eggs and flour along with the poppy seeds and vanilla. Mix well until everything is combined.
Spoon the mixture into the lined loaf tin and then pop it into the oven for 50–55 minutes, or until golden brown and a skewer inserted into the centre of the cake comes out clean.
When the cake is cooked, remove it from the oven and leave it to cool in the tin, then cut into slices and serve.