Sift icing sugar to remove lumps. Set aside.
In a clean bowl, cream butter until smooth (with a paddle attachment). Add icing sugar, a little at a time and mix until well combined.
The mixture may appear crumbly at this point and that is perfectly fine.
Add condensed milk, mint extract and green food color and continue beating. If the mixture is still crumbly, try pressing the mixture with your fingers, if it comes together, it is ready for rolling and cutting.
Continue kneading by hand until the mixture comes together into a ball and the color is well distributed.
Scoop the mixture with a 1 tablespoon measuring spoon. Roll it to form a ball and then flatten it with your fingers. Place a chocolate butter in the center and carefully cover the chocolate from the sides with the flattened fondant mixture.
Once the chocolate is completely covered, press the patty in between your palms to flatten and smooth the mint mixture. Place the chocolate filled mint patty on a parchment lined tray. Continue until all the mixture has been used up to make the patties.
Let the patties rest at room temperature overnight. The next day, turn them over and let them set again for another day until they have dried and set well. Store the mints in a candy jar in a cool and dry place.