Preheat the oven to 325 degrees.

Prepare the pans. Line 2 baking sheets with parchment paper or silicone baking mats.

Whisk together the flour, baking soda, and salt in a medium sized bowl.

In a stand mixer, beat together the butter and sugars on medium until smooth and combined, about 1-2 minutes.

Add the egg, yolk, and vanilla, and beat again until combined, scraping the bowl as needed.

Add the flour mixture and mix on low speed until just combined.

Stir in 1 cup of the mini eggs and the walnuts, if using.

Using a Tablespoon sized cookie scoop, scoop heaping Tablespoon sized balls onto the parchment lined baking sheets, leaving at least 2 inches of space between them.

Bake until the edges are set, but the centers are still very soft and puffy, 11-13 minutes. As soon as they come out of the oven, gently press the remaining mini eggs on top. Let the cookies cool on the baking sheet until they can be moved, about 5-7 minutes, then transfer to a wire rack to finish cooling.