Heat oven to 350 degrees. Spray a 9x13 baking pan with cooking spray and then line with parchment paper. The cooking spray will help the parchment paper stay in place. Or, just use cooking spray, or line the pan with tin foil and spray it lightly.
In a small mixing bowl combine flour, salt, and baking soda. Stir with a whisk to break up clumps
Slowly add in the flour mixture and mix until combined and no flour pockets remain in the batter. Add the chopped Cadbury Mini Eggs and mini chocolate chips, reserving some for sprinkling on top if wanted. Stir to combine.
Dump batter into the prepared pan and spread out evenly. Sprinkle the reserved Cadbury Mini Eggs and mini chocolate chips over top and gently press down.
Bake for 21-26 minutes. The edges will be a light golden brown and be "wrinkly" and the middle will look pale. Let cool completely in pan on a cooling rack.
To help the butter be slightly cooled, simply melt the butter in the microwave until it's only half melted, about 40-45 seconds. Use a fork to stir together and that should finish melting the butter. You'll be left with slightly cooled melted butter that can be used right away.