Preheat the oven to 350°F and line a large baking sheet with parchment paper and set aside. If you only have a 9x13-inch baking sheet, you'll want to use two baking sheets for two batches.
In a medium bowl, whisk together the flour, cocoa, malted milk powder, cornstarch, salt, and baking powder. Set aside.
1½ cup all-purpose flour, ½ cup Dutch-processed cocoa powder, 1 tablespoon malted milk powder, 2 teaspoons cornstarch, ½ teaspoon salt, ½ teaspoon baking powder
Partially melt the butter in a microwave-safe bowl. I melt mine for 45 seconds directly from the refrigerator. You want the melted butter to just be pooling a bit but still have a decent amount of stick left.
½ cup unsalted butter
Add the butter and sugars to a large bowl or stand mixer fitted with a paddle attachment and cream just until smooth.
½ cup light brown sugar, ½ cup granulated sugar
Add in the egg and vanilla extract and beat until fully incorporated.
1 large egg, 1 teaspoon pure vanilla extract
Add half of the dry ingredients to the wet ingredients and beat just until incorporated. Then add the remaining dry ingredients and mix until thoroughly combined.
Portion out the dough into 8 even portions (about 2.9 oz. each) and gently press your thumb into the middle of the ball to create a well. Place an egg in the well and gently use your hands to work the dough up and around the sides and top of the egg and pat and pinch to seal. Roll the ball between your palms to make it smooth and round.
8 Cadbury Creme Eggs
Place cookie dough at least 2 inches apart on the prepared baking sheet. Bake for 8 to 10 minutes until the edges are just set.
Allow the cookies to cool on the pan for 15 minutes before transferring them to a wire rack with a spatula to cool completely, about 1 hour more. These cookies are very soft right after baking and will firm up while they cool.
Once cooled, melt the chocolate melting wafers in the microwave at 30-second intervals, stirring between each one until fully melted. Drizzle over the tops of the cookies and then top with holiday sprinkles and a mini Cadbury creme egg if desired. Let set at room temperature until the chocolate hardens, about 15 to 30 minutes.
6 ounces chocolate melting wafers, 1 tablespoon holiday sprinkles, 8 mini Cadbury Creme Eggs