Instructions

• Preheat the oven to 400°F. Line baking sheets with parchment paper.
• In a medium bowl, whisk together 2 1/2 cups all-purpose flour, 1 1/2 teaspoons baking soda, and 1/2 teaspoon salt; set aside.
• In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, cream 1/2 cup butter-flavored shortening and 1 cup granulated sugar until light and fluffy.
• Add 1 egg, 2 teaspoons vanilla extract, 1/2 cup buttermilk, and 1/2 cup hot water to the creamed shortening and sugar. Mix until fully incorporated, scraping the bowl as needed.
• With the mixer on low, slowly add the dry ingredients to the wet ingredients and mix until just combined. Do not overmix.
• Gently fold 1 cup Fruity Pebbles (lightly crushed) into the batter with a rubber spatula.
• Use a medium cookie scoop (about 1.5 tablespoons) to drop rounded portions of batter onto the prepared baking sheets, spacing them about 2 inches apart.
• Bake the cakes for 8–9 minutes, until set. Remove from the oven and let the cakes cool on the baking sheets for 5 minutes, then transfer to wire racks to cool completely.
• While the cakes cool, make the filling: in the bowl of a stand mixer fitted with the paddle attachment, beat 3/4 cup unsalted butter (room temperature) on medium-high speed for 3–4 minutes until pale and creamy.
• Turn the mixer to low and gradually add 1 1/4 cups sifted powdered sugar until incorporated. Increase the mixer to medium-high and beat for 5 minutes.
• Add 2 teaspoons vanilla extract and the entire 7-ounce container of marshmallow cream. Beat for an additional 2 minutes until the filling is smooth and spreadable. Scrape the bowl as needed.
• To assemble, place half of the cooled cakes flat-side up. Spoon or pipe an even amount of the marshmallow buttercream onto each of those cake halves.
• Top each filled cake with the remaining cake halves, flat sides together, pressing gently to spread the filling to the edges.
• Place the remaining 1/2 cup Fruity Pebbles (lightly crushed) in a shallow bowl or on a plate. Roll the edges of each assembled whoopie pie in the Fruity Pebbles, pressing gently so they adhere.