Preheat the oven to 350°F. Grease and line a 9×13 metal baking pan with parchment paper.

In a stand mixer (or using a hand mixer or whisk), beat the oil, granulated sugar, brown sugar, and eggs on medium-high speed until light, about 3 minutes. Add vanilla and melted butter, and beat for 30 seconds.

Add the flour, cocoa powder, and salt to the wet ingredients and mix lightly until just combined. Gently fold in the chocolate chips and Cadbury Eggs.

Pour the batter into the prepared pan and spread evenly. Bake for 22-25 minutes, or until the center no longer jiggles. For fudgy brownies, do not overbake! For cakey brownies, bake for 5 minutes longer. Remove from the oven and let cool completely before frosting.

In a stand mixer (or using a hand mixer), beat the butter, powdered sugar, and vanilla extract on medium speed until smooth. Add the salt and 1 tablespoon of milk at a time until you reach the desired consistency. Add a small amount of blue food dye, adding more until you reach the desired color. Spread immediately on the cooled brownies.

In a small bowl, combine warm water and cocoa powder. Whisk until combined. Once the brownies are frosted, dip the tip of a dry pastry brush or paintbrush into the water and cocoa mixture and shake off any excess. Pull the bristles back, point at the brownies, and slowly release. Repeat as many times as you'd like to create the pattern. Sprinkle the chopped Cadbury Eggs on top. Slice and serve!