Preheat oven to 350°F, line a metal 9x13-inch cake pan with parchment paper, and spray lightly with cooking spray. Ensure some of the parchment is over the edge of the pan for easy removal. This may require two sheets, one going in each direction of the pan - make sure the one that goes across the shorter distance is under the one that goes along the entire length of the pan.
Melt 1 cup salted butter in a heavy-bottomed saucepan over low heat. Add the 2½ cups granulated sugar and stir until the butter and sugar have started emulsifying and no butter is separated on top of the sugar. Remove from heat and let the mixture cool for 5 minutes, stirring often to release heat.
Add 2 tablespoons canola oil and 1 tablespoon vanilla extract to the butter and sugar and beat them in with a rubber spatula until fully combined and there's no separation between the ingredients (no oil resting on top).
Mix in 5 large eggs one at a time, beating with the spatula or a fork after each one. Mix until fully incorporated.
Sift the ¾ cup + 2 tablespoons all-purpose flour, ¾ cup + 1 tablespoon unsweetened cocoa powder, and 1 teaspoon fine sea salt into the butter and sugar mixture.
Stir the dry ingredients in gently until almost all the dry streaks are gone.
Roughly crush or chop 1½ cups Cadbury Mini Eggs (you don't need to crush every egg; some whole in the brownie is just fine). I found crushing them like you would a garlic clove with the side of a knife and pressing it down on them worked pretty well. But you can only do a few at a time.
Fold the chopped eggs into the brownie batter.
Pour the brownie batter into the prepared baking pan and bake for 28 to 32 minutes.
Remove from the oven and allow the brownies to cool completely before adding the next layers. Lift the brownies out of the pan using the edges of the parchment paper before frosting.