In a large bowl, beat together the butter, brown sugar and white sugar until fluffy (about 2 minutes).
Beat in the egg, egg yolk and vanilla extract until smooth.
Add in the flour, baking soda and salt. Stir a few times with a rubber spatula, then mix with your electric mixer on a low speed until combined.
Fold in ¾ cup chocolate chips and sprinkles using a rubber spatula or wooden spoon. The batter will be thick.
Cover the bowl and refrigerate for an hour, or up to 24 hours.
When ready to bake, preheat the oven to 350 F (180 C or 160 C on a fan-forced oven). Line cookie sheets with baking paper or baking mats.
Form the dough into balls with about 1 - 1.5 tablespoons of dough each (a little smaller than a golf ball). If the dough has been refrigerated overnight, let it sit on the counter for 10-15 minutes first. Place the dough balls 2 inches apart on the cookie sheets.
Bake 1 sheet at a time in the middle of the preheated oven for about 11-13 minutes, or until the tops look just set.
Remove from the oven and place a few chocolate chips on the top of each cookie. Cool on the cookie sheet for about 10 minutes, then transfer to a cooling rack to continue cooling.