Preheat oven to 375˚F (190˚C). Line a muffin pans with 12 paper liners.
In a food processor, add sugar and mandarin orange zest. Pulse until the zest is smaller and the sugar is yellow in color.
In a large mixing bowl, whisk together flour, baking soda, baking powder and salt. Set aside.
In a medium mixing bowl, whisk together eggs, sour cream, warm milk, melted butter, mandarin orange juice and sugar until combined.
Add the wet ingredients into the dry ingredients. Whisk until just combined. Small lumps are ok. Fill the cupcake liners about 2/3 filled. Bake for 5 minutes. Reduce the temperature to 350F and bake for 10 – 12 more minutes. Test the cupcakes with a toothpick for doneness. The toothpick should come out clean. The cupcake should spring back when press lightly. Remove from oven and cool in the pan for 5 minutes. Transfer the cupcakes to a cooling rack and cool completely.
For the frosting, in a large mixing bowl with an electric mixer fitted with paddle attachment, beat the cream cheese until smooth. Add the mandarin orange juice, zest, vanilla extract, powdered sugar and food colorings at low-speed until combined. Add the heavy cream and whip until the frosting is thick enough to use. Do not over beat.
To decorate the cupcake, preparing a piping bag with a ½-inch round tip. Fill the bag with frosting. Hold the pipping bag up straight and pipe from the center, squeeze to create a dome of frosting. To increase the diameter of the dome, insert the pipping tip into the dome and squeeze out more frosting. Smooth out the very top of the frosting with an offset spatula.
For the optional chocolate decorations (stems and leaves), place the white chocolate in a medium heatproof bowl. Heat over a pot of barely simmering water until it is melted. Take it off the heat once it is melted, add food colorings. Use it immediately. If the chocolate starts to harden again, put it back over the simmering water to melt.
To make the stems, cut the straws into 12 (1-inch) small tubes. Cut them open lengthwise. Even though straws are slit opened, they will curl back to the original position. Put some of the melted chocolate into a pipping bag and cut a small hole (that can fit into the straw). Squeeze the chocolate into the straws. Chocolate may leak out and it can be messy, but don’t worry. Just wipe them off after. Chill in the fridge for at least 20 minutes to set.
To make the leaves, cut paper towel tubes lengthwise and open them up a little. These will be used as sharping tools. Set aside. Prepare acetate sheets. Cut them into 1x2½-inch pieces. Dip the tip of a paring knife into the melted chocolate. Place the blade of the knife on an acetate piece, gently press down and pull toward you. Carefully turn the knife to the back and pull toward you again to create the stem part. If the leaf looks too thin, patch with a little extra chocolate. Lay the acetate piece inside the prepared paper towel tube to set (chocolate facing up). Repeat and make about 15 - 17 leaves. Chill in the fridge for at least 20 minutes to set.
When the chocolates are ready, carefully unroll the straws and use the back of knife to push out the stems. Gently peel the leaves from acetate sheets. Place them on the frosted cupcakes. If you have warm hands, you may want to cool them down with cold water or ice cubes first, but dry your hands before touching the chocolate.
Cupcakes are best served on the same day.