Combine butter, mini marshmallows, and salt in a large pot over medium heat.

Heat the mixture while stirring constantly until butter and marshmallows are melted and a smooth consistency.

Remove from heat and fold in the Rice Krispies cereal until full incorporated into marshmallows.

Press mixture into a rimmed sheet pan lined with parchment paper.

Quickly before the Rice Krispy Treats cool, press Cadbury mini egg pieces into the treats and top with sprinkles if desired.

Let the Rice Krispy Treats sit and solidify for at least an hour before cutting and serving.