Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, cream together the butter, brown sugar, and sugar for 4 minutes, scraping the bowl as needed. This is key for a smooth, fluffy base.
Add in the egg and vanilla extract, beating until fully combined.
In a separate bowl, whisk together the flour, baking soda, and salt. Gradually fold this mixture into the wet ingredients until just combined.
Gently stir in mini Cadbury eggs (reserving 1/3 cup for the tops) and any optional chocolate chips. Some of the Cadbury eggs will naturally break apart—this adds great texture!
If possible, wrap the dough in plastic wrap and chill for at least 30 minutes (or up to 48 hours) in the fridge. This step enhances the cookie's flavor and structure.
Scoop the cookie dough into balls and place them on the prepared baking sheet. Leave 2 inches of space between each ball.
Chop the reserved Cadbury eggs in half and gently press them into the tops of the dough balls.
Bake for 9-10 minutes, or until the cookies turn a light golden-brown color around the edges. For crispier edges, bake 1-2 minutes longer.
Remove from the oven and let the cookies cool on the sheet for 5 minutes before transferring to a wire rack to cool completely.