Start by beating the butter, brown sugar, and granulated sugar together until smooth and creamy. This step is important because proper creaming creates a soft texture. Don’t rush it — mix until light and fluffy.
Add the egg and vanilla extract to the creamed mixture. Mix until fully combined. Scrape down the sides of the bowl to make sure everything blends evenly.
In a separate bowl, whisk together flour, cocoa powder, baking soda, espresso powder, and salt. Whisking ensures the cocoa and baking soda distribute evenly, preventing lumps.
Slowly add the dry mixture to the wet mixture. Mix on low speed until just combined. Avoid overmixing, as that can make the cookies tough instead of soft and fudgy.
Gently fold in the chopped mini eggs and dark chocolate chunks. The dough will be thick and slightly sticky — that’s exactly what you want for brownie-style cookies.
Wrap the dough in plastic wrap and chill for 30–45 minutes. Chilling prevents excessive spreading and keeps the cookies thick and chewy.
Preheat the oven to 350°F (175°C). Scoop dough onto a lined baking tray, leaving space between each cookie. Sprinkle flaked sea salt on top. Bake for 10–12 minutes until the edges are set but centers are slightly soft.
Let cookies cool on the baking tray for 5 minutes before transferring to a cooling rack. They continue to set as they cool, giving you that perfect fudgy center.