Make the Fruity Pebbles Treats: Line an 8” x 8” (20 x 20 cm) baking pan with parchment paper and grease lightly with a nonstick baking spray.
2. Step
Melt the butter in a medium pot over medium-low heat. Once melted, add marshmallows and salt, stirring frequently until completely melted and butter is incorporated, about 3–5 minutes. Remove from heat.
3. Step
Add Fruity Pebbles cereal and stir to combine, until the marshmallow mix is distributed evenly. Spread in parchment-lined pan into an even layer, pressing down with a greased spatula, and let cool completely.
4. Step
Once cooled, remove the Fruity Pebbles treat from the pan by lifting the parchment paper out and set on a cutting board. Cut into ½-inch (1.2 cm) squares.
5. Step
Make the Skillet Cookie: Preheat the oven to 350°F (180°C).
6. Step
In a large bowl, cream together the butter and brown sugar with an electric hand mixer on high speed until light and fluffy, 2–3 minutes. Add the vanilla, then add the egg and egg yolks, 1 at a time, beating on medium speed between each addition to combine. Scrape down the sides of the bowl with a spatula as needed.
7. Step
In a medium bowl, whisk together the flour, baking soda, baking powder, and kosher salt.
8. Step
Add the dry ingredients to the large bowl with the wet ingredients and mix on medium-low speed until just combined. Add the Lucky Charms cereal and mix on low speed until just evenly distributed, about 30 seconds.
9. Step
Grease a 10-inch cast-iron skillet with nonstick cooking spray. Spread about one-third of the cookie dough evenly into the skillet, using a spatula to smooth out the dough. Add half of the Fruity Pebble Treat squares and press them down into the cookie dough. Spread the remaining two-thirds of the dough over the squares and smooth out as evenly as possible with a spatula.
10. Step
Bake the cookie for 35–40 minutes, until the top is golden brown and the center is no longer jiggly when the pan is shaken.
11. Step
Immediately after the cookie is removed from the oven, gently press the remaining Fruity Pebble Treat squares into the top of the cookie. Scatter Lucky Charms marshmallows over the top and carefully press those into the cookie surface. Let the cookie cool in the pan for 20 minutes.
