In a bowl, stir together the melted butter and sugar until smooth. Add the vanilla, egg and egg yolk, and stir until smooth.
Separately, combine the flour, corn starch, salt, baking soda and cardamom. Add the dry ingredients and stir just until the flour is incorporated into the dough. Let cool for 10 minutes.
Fold in the white chocolate chips and the sprinkles (reserving 1 tablespoon sprinkles for later); the dough will be thicker now than before it cooled.
Portion the dough into 20 equal portions. Roll into balls, and slightly flatten each ball. Sprinkle the top of the dough balls with the reserved sprinkles.
Cover the dough balls with plastic wrap and refrigerate for 30 minutes.
Preheat the oven to 350 F, and position a rack in the center of the oven.
Line a baking sheet with parchment paper, and place the chilled dough 3 inches apart on the baking sheet. Refrigerate the unbaked dough while you bake each batch of cookies.
Bake the cookies for about 9 minutes, until a pale golden brown, and the centers are nearly set, but ever so slightly underdone. Cool on the baking sheet for 1 minute, then transfer to a cooling rack to cool completely.
Bake the remaining dough. You may need to bake the last batch for about 30 seconds longer, since it had a longer chill time.
Store the cooled cookies in an airtight container for up to 3 days, or freeze for up to 3-6 months.