Special equipment: 12-cup muffin or cupcake pan
Preheat the oven to 350 degrees F.
Line a 12-cup muffin or cupcake pan with 12 paper liners.
In a large bowl, or in an electric food mixer, cream the butter until soft. Add the sugar and 1/2 the 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 butter mixture. Alternatively, you can whiz or pulse all the ingredients together in an electric food mixer.
Divide the mixture evenly among the paper liners, and bake in the preheated oven, until the cupcakes have risen and are golden, about 7 to 10 minutes. When cooked, the center of each cake should be slightly springy to the touch. Remove the cakes from the pan, and place on a wire rack to cool completely before icing.
To make the lemon butter icing: In a bowl, using a wooden spoon or hand-held electric beater, cream the butter until very soft. Gradually add the sugar, and beat into the butter, along with the remaining 1/2 of the lemon zest and enough lemon juice to soften the icing to a spreadable consistency.
When the cupcakes are cool, spread a generous heaping teaspoon of lemon butter icing over the top of each one. Decorate the cupcakes with the crystallized flowers, if using.