Heat oven to 180C/160C fan/gas 4.
Butter then flour a 1.5-litre Bundt or ring
tin. Beat together the butter and sugar
until pale and fluffy. Beat in the eggs, one
at a time, followed by the vanilla, flour,
baking powder, soured cream, grapefruit
zest and juice, and toasted poppy seeds.
Scrape into the tin and level it off. Bake
for 40-55 mins, depending on the shape
of your tin, until a skewer poked in comes
out clean. Leave in the tin to cool a little
while you make the icing.