Prior to starting, unwrap creme eggs and refrigerate until solid, at least 2 hours.

Line two large baking sheets with parchment paper, and set aside.

Stir together brownie mix, flour, butter, egg, and espresso powder (if using) in a large bowl until just combined into a stiff dough that resembles the texture of Play-Doh.

Portion cookie dough into 10 even portions (about 3 oz./57 g each). Working with 1 portion at a time, flatten dough in palm of your hand to form an even round. Place 1 chilled creme egg in center of dough round, and wrap dough to fully enclose the creme egg, rolling it around in your palms to create a smooth ball. Vinyl kitchen gloves are really helpful for this stage. Place on prepared baking sheet. Repeat with remaining creme eggs and dough portions, ensuring that cookie dough balls are placed at least 2 inches apart on each baking sheet (5 per baking sheet).

Chill formed cookies on sheet pans in freezer for about an hour. When the hour is nearly up, preheat oven to 375 degrees F (190 degrees C) with rack in top third position. Allow oven to fully preheat before baking.

Bake one baking sheet at a time in preheated oven until cookies are just set on the edges and on top, 10 to 11 minutes. Do not overbake or the creme eggs may seep out of the cookie. Let cool on baking sheet slightly, about 15 minutes. Serve warm, or let cool completely on a wire rack and store in an airtight container for up to 2 days.