Line a 12-hole muffin tin with paper cases and heat oven to 180C/fan 160C/gas 4. In a large bowl, cream together the butter, sugar and lemon zest until pale and fluffy. Add the polenta and continue to whisk until combined. Beat in the eggs, a little at a time.
Sift in flour and baking powder, then fold in quickly with a large spoon or spatula until you have a thick batter (the mixture may look lumpy because of the polenta). Stir in the milk to loosen, then gently fold in the chopped strawberries. Divide the mixture between the paper cases, then bake for 20 mins or until golden, risen and springy to touch. Cool for a few mins, then place the cakes on a wire rack to cool completely. The cakes can now be stored in an airtight container for up to two days.
Carefully peel the cases off the cakes. Sift the icing sugar into a large bowl. Place the chopped strawberries in a bowl with 1 tsp lemon juice and mash with a fork until pulpy. Push through a sieve over the icing sugar (you only need a few drops of juice to colour the icing a rosy pink). Stir in more lemon juice, a dribble at a time, until you have a thick yet fluid paste. Dip the top of each cake into the icing, then top immediately with a strawberry half. Leave to set, then serve.