In a bowl add butter brown sugar, egg yolk, add vanilla extract and beat it. In a small bowl, add flour, milk powder, masala chai powder, baking powder and soda. Mix it and sieve it well. In the bowl containing butter and sugar mixture, add dry ingredients and beat it. Make dough and shape it into small round tikis of same size. Roll it in oats. Bake it in a pre heated oven at 180 C for 15 20 min. Masala chai cookies are ready to serve.