Preheat the oven to 180ºC (350ºF), Gas mark 4. Line a 12-hole cake tin with 12 paper cases.
Cream the butter in a large bowl or in an electric food mixer until soft. Add the sugar and grated lemon zest and beat until the mixture is light and fluffy. Gradually add the beaten eggs, then sift in the flour and baking powder and fold into the mixture. Alternatively, whiz all the ingredients together in an electric food mixer.
Divide the mixture between the paper cases and bake in the preheated oven for 7–10 minutes, until risen and golden. When cooked, the centre of each cake should be slightly springy to the touch. Remove the cakes from the tin and place on a wire rack to cool before you ice them.
To make the lemon butter icing, cream the butter in a bowl with a wooden spoon or hand-held electric beater until very soft. Gradually add the icing sugar and beat into the butter, along with the lemon zest and enough lemon juice to soften the icing, to a spreadable consistency.
When the cupcakes are cool, spread a generous heaped teaspoon of lemon butter icing over the top of each one. Decorate with the crystallised flowers (if using).