In a mixing bowl, combine the powdered sugar, unsalted butter, milk, and vanilla extract. Mix until smooth and creamy, ensuring there are no lumps.
Carefully fill each maraschino cherry with the prepared filling, leaving some space at the top. Set them aside on a parchment-lined tray.
Place the tray of filled cherries in the refrigerator for about 30 minutes to set the filling.
Melt the dark chocolate using a double boiler or microwave in short bursts, stirring in between, until smooth and glossy.
Dip each chilled cherry into the melted chocolate, ensuring they are completely coated. Allow excess chocolate to drip off before placing them back on the parchment paper.
Return the dipped cherries to the refrigerator for an additional 30 minutes to allow the chocolate to set completely.