Preheat the oven to 190°C. Place the sifted flour, sugar, softened butter, lemon zest and eggs into an electric mixer bowl and mix on slow speed for 1 minute until combined. Pour in the milk and continue to mix on kneading speed for 30 seconds. Turn up the speed and mix for 3 minutes until fluffy and smooth. Line a 9-hole muffin pan with the cupcake cases. Using a piping bag, fill with the cupcake batter and use to just over half fill the cases. Bake in the oven for 20-25 minutes until golden and puffed and spongy to the touch. Meanwhile, make the lemon butter cream. Place the softened butter, icing sugar and lemon zest into the mixer bowl and mix on slow speed for 1 minute until combined. Add in the lemon curd and continue to mix on fast speed for 2 minutes until the butter cream is light and fluffy. As soon as the cupcakes have completely cooled down, Use the end of a small spoon to make a small cavity in the center of each cupcake. Place 3 tablespoons of lemon curd into a piping and fill the cavity in each cup cake with lemon curd. Next, place the lemon butter cream in a piping bag and pipe a decorative swirl on top of each cup cake. Sprinkle over the extra lemon zest and serve.