Preheat the oven to 350℉ (180℃). Grease a 9x9 inch (23x23 cm) square baking pan and line it with a long 9-inch wide strip of parchment paper that hangs over two of the sides.
Set aside 20 mini eggs (5 of each colour). Place the rest of the mini eggs into a zip-top plastic bag and crush them lightly by pushing down on them with the bottom of a glass or small pot just until they crack. This will make the bars easier to cut. Set them aside, also.
Cream the butter, brown sugar, and honey until they are fluffy and light-coloured, either with an electric mixer or by hand.
Add the egg, vanilla, salt, and nutmeg or cinnamon (if using), and beat until creamy.
Add the baking soda and flour, and mix until the flour is all incorporated. You will have a medium-soft cookie dough.
Stir in the crushed mini eggs.
Scrape the dough into the prepared baking pan and push it down evenly with your fingers. Scatter the 20 whole mini eggs over the cookie dough and press them gently down into it.
Bake for 18-20 minutes, until the edges are golden and the center is dry but still quite soft to the touch. It will firm up as it cools, but that is what will make the cookie bars slightly fudgy.
Allow to cool in the pan for 20 minutes, then gently lift up the whole block of cookie bars by the two paper flaps and transfer it to a cutting board. Cut the bars while they are still slightly warm into 16 pieces.
Notes
If you don't have enough mini eggs, you can replace ½ to ¾ cup of the crushed mini eggs with milk chocolate chips instead.