In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

In a large bowl or stand mixer, beat the softened butter, brown sugar, and granulated sugar on medium-high speed until pale and fluffy (about 2-3 minutes).

Add the egg, egg yolk, and vanilla extract. Beat until smooth and well combined, scraping down the sides of the bowl as needed.

Turn the mixer to low speed and gradually add the dry flour mixture. Mix just until combined. Do not overmix.

Gently fold in the chocolate chips and rainbow sprinkles using a spatula.

Scoop about 2.5 to 3 tablespoons of dough. Flatten it into a disc in your hand, place one Peep in the center, and carefully wrap the dough around it. Roll into a smooth ball, ensuring the Peep is completely sealed inside with no cracks.

Place the stuffed dough balls on a baking sheet and chill in the refrigerator for at least 30 minutes.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

Space the chilled dough balls at least 2 inches apart on the baking sheet. Bake for 11-13 minutes, or until the edges are golden brown.

Let the cookies cool on the baking sheet for 10 minutes before transferring to a wire rack. This allows the gooey center to set slightly.