Preheat oven to 350 degrees F. Line an 8-inch square baking pan with non-stick aluminum foil, making sure to leave enough overhang on the sides. Set aside.
In a stand mixer, beat butter and sugar on medium speed until light and fluffy. Beat in egg, egg yolk, and vanilla. Scrape down the sides as needed.
In a medium bowl, combine flour, salt, baking powder, baking soda, and cornstarch. With mixer on low, gradually add flour mixture to wet ingredients and beat until combined. Gently fold in 1/2 cup of the Fruity PEBBLES™ cereal using a rubber spatula.
Press the cookie dough evenly into the prepared baking pan. Bake for 20-25 minutes or until very lightly browned on top. Cool completely on a wire rack.
Spread ice cream evenly over cookie base in the baking pan. Top with remaining 1/2 cup of cereal. Lightly press cereal into ice cream. Cover with plastic wrap and freeze until set, at least 2 hours.
When ready to serve, lift foil out of pan and cut into 16 2-inch squares.