Cupcakes:

1. Preheat the oven to 350 ° F. Line muffin tins with paper liners (find St. Patrick's Day liners, or use Green or White liners).

2. Sift the flour, baking soda, and salt into a bowl and set aside.

3. In a stand mixer fitted with the paddle attachment, cream together the butter, sugar, and lemon zest on medium speed, scraping down the bowl with a rubber spatula as needed, until the mixture is smooth and light in color, about 5 minutes. Add the eggs one at a time, beating well and scraping down the bowl after each addition.

4. Alternate adding the sifted dry ingredients and buttermilk to the creamed mixture, mixing on low speed until just incorporated. Increase the speed to medium and mix until very light and smooth, another 2 minutes. Add the lemon juice last and blend just until evenly mixed, 30 seconds.

5. Fill the muffin tins; each cup should be filled two-thirds full. Bake until the cupcakes rise in a dome shape and spring back when lightly pressed with fingertip, 16-20 minutes. Place the cupcakes in their pans on a wire rack to cool completely before removing from the tins.

Icing:

1. Chop the white chocolate and melt in the microwave on low power in short bursts, about 15 seconds each, until it is barely softened. Stir the chocolate until it is smooth, returning it to the microwave if necessary. (White chocolate can overheat quickly so keep power low and the time short. Alternatively, melt the white chocolate over barely simmering water.) Let the chocolate cool to room temperature.

2. Cream butter and cream cheese with the paddle on medium speed until very smooth and light, about 3 minutes. Scrape down the bowl to blend the icing evenly. Add chocolate, vanilla, and salt; continue to mix until very smooth, another 1-2 minutes.

3. Add green food coloring to the icing, in celebration of St. Patrick's Day. Add green sprinkles and decoration as desired.