In a small bowl, combine warm water, yeast, and sugar. Stir gently and let sit for 5-10 minutes until frothy.
In a larger mixing bowl, combine warmed milk, yogurt, olive oil, and egg yolk. Mix well.
In another bowl, whisk together flour and salt.
Add the yeast mixture to the wet ingredients, then gradually incorporate the dry flour mixture. Stir until a shaggy dough forms.
Transfer the dough onto a floured surface and knead for 5-7 minutes until smooth and elastic.
Place the dough in a greased bowl, cover with a damp cloth, and let rise in a warm place for 1-2 hours until doubled in size.
Punch down the dough and divide into 6-8 pieces. Roll each into a ball and flatten into tear-shaped discs.
Preheat a skillet over medium-high heat. Cook the naan for 1-2 minutes on each side until bubbly and golden.
Brush with melted butter and serve warm.