Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.
In a medium-sized mixing bowl, whisk the flour, salt, baking soda, and cornstarch.
In a large mixing bowl, add the butter, brown sugar, and sugar. Beat until light and fluffy, 1 to 2 minutes. Add the milk and vanilla and beat again until smooth and creamy.
Add about half of the flour mixture to the wet mixture and use a rubber scraper to mix until combined. Add the rest and mix until just combined.
Fold in the chocolate chips (if using).
Place sprinkles in a small bowl (start with 3/4 cup, you may end up needing more or less).
For each cookie, roll out ~1 tbsp (15 g) of dough and coat in the sprinkles. Place on the prepared baking sheets. Leave about 2 inches (5 cm) in between each ball.
Bake in the preheated oven for 8-10 minutes, until puffy. They may look slightly underbaked, but will bake more while cooling - don't overbake them.
Allow to cool on the baking sheets then enjoy!