In a large mixer bowl, mix together glutinous rice flour, plain flour, sugar, and salt.
Add in warm water and mix the ingredients using a spatula until it gets crumbly.
Add in butter and gently knead the dough until you get a smooth and clay-like texture.
Cover the mixer bowl with cling film and let the dough rest for 30 minutes.
Divide the dough into 10 equal pieces.
Working on one piece at a time, use your thumb to gently press the dough and push out from the centre. Wrap the mochi dough around a mozzarella cheese cube.
Roll the dough into balls.
In a pot, heat the oil to 180°C and transfer the dough balls into the pot.
Fry the balls for about 10-15 minutes until golden brown, constantly flipping and moving the balls around to cook them evenly.
Remove the cheese balls from the pot and let the oil drain on a wire rack before serving.