In a medium saucepan, whisk together egg yolks, sugar, and cornstarch until smooth. Gradually pour in the milk while whisking to combine. Place over medium heat, stirring constantly for about 5-7 minutes until thickened.

Stir in the butter and vanilla extract until melted. Transfer custard to a bowl, pressing plastic wrap onto the surface. Chill in the refrigerator for at least 30 minutes.

In a large mixing bowl, combine glutinous rice flour and sugar. Gradually add water or milk while stirring until smooth. Microwave mixture in 1-minute intervals, for about 3-4 minutes until thick and sticky.

Dust hands and surface with cornstarch. Divide mochi dough, flatten into discs, place custard in center, wrap dough around it and pinch to seal. Roll into a ball.

Roll mochi balls in crushed vanilla cookies and place in mini cupcake liners. Chill in the refrigerator for 20-30 minutes.