Begin by melting 3 tablespoons of butter in a large saucepan over low heat. Once melted, add 4 cups of mini marshmallows, stirring continuously until completely melted and smooth.

Remove from heat and mix in 6 cups of Rice Krispies, ensuring every piece is coated in the marshmallow mixture. Allow the mixture to cool slightly until it's manageable.

Once the Rice Krispies mixture is cool enough to handle, press it into a greased baking dish or onto a lined surface to form a flat layer about an inch thick. Using an egg-shaped cookie cutter, cut out egg shapes and gently transfer them to a baking sheet lined with parchment paper. Allow them to set at room temperature for about 30 minutes.

In a medium bowl, combine 4 cups of powdered sugar with 3 tablespoons of meringue powder. Gradually add about 5 tablespoons of water, mixing with an electric mixer on low speed until the icing is smooth and glossy.

Divide the royal icing into several small bowls, depending on how many colors you want. Add different food colorings to each bowl, mixing well to achieve vibrant pastel shades.

Transfer the colored royal icing into squeeze bottles for better control. Start by decorating each Rice Krispies egg with fun designs; allow the decorated eggs to sit at room temperature for about 1-2 hours, or until the icing hardens completely before serving.