Add the butter, brown sugar, and granulated sugar to a large bowl. Beat with an electric mixer on medium-high for 2 minutes.
Add the eggs and vanilla to the butter and sugar mixture and beat on low until just combined.
Preheat the oven to 350 degrees F. Line a 9x13 inch baking dish with parchment paper.
In a medium bowl, whisk together the flour, baking soda, and salt. Add the flour mixture to the wet ingredients. Use a sturdy spoon to mix together until just combined.
Add 1 ½ cups of chocolate eggs to a ziploc bag, seal, and beat with a rolling pin or a similar object to break the eggs into pieces. Fold about half of the broken chocolate egg pieces into the dough. Reserve the rest for the top.
Using dry, clean hands, press the dough into the prepared baking dish. Bake for 15 minutes, then remove from the oven and lighlty press the remaining broken chocolate egg pieces and ½ cup of whole chocolate eggs into the top of the cookie bars. Bake an additional 7-10 minutes, or lightly until golden brown around the edges, but still a little doughy in the center. The cookie will continue to cook in the warm baking dish outside of the oven.
Let cool for 1 hour. Using the edges of the parchement paper, lift the cookie out of the baking dish and cut into squares.