In a mixing bowl, combine sweetened condensed milk and cocoa powder until it forms a smooth and thick chocolate mixture. Refrigerate to firm it up while preparing the wrapper.
In a large mixing bowl, sift together glutinous rice flour, rice flour, wheat starch, and powdered sugar. Pour in milk, condensed milk, and oil. Whisk well.
Transfer the mixture to a heat-proof bowl and steam on medium heat for 25 minutes.
Once the dough has cooled slightly, knead it until smooth and pliable.
Divide the dough into 25g portions and flatten each in your palm.
Place 25g of chocolate truffle filling in the center, fold and gently press to seal.
Dust with glutinous rice flour to prevent sticking.
Press each dough ball into a mooncake mold to create the desired pattern. Release from the mold and place on parchment paper.
Repeat for the remaining mooncakes.
Refrigerate for at least 2 hours before serving.