Prep: Preheat oven to 190°C (375°F). Line baking sheets with parchment.
Brown butter: In a light colored saucepan over medium heat, melt butter, swirling occasionally. Cook until amber with brown specks and nutty aroma. Immediately remove from heat and pour into a large bowl. Refrigerate until completely cold, about 30-40 minutes.
Mix wet: To cold brown butter, add brown sugar and granulated sugar. Beat for 2-3 minutes until smooth. Add egg and vanilla, beat until creamy and glossy.
Mix dry: In a separate bowl, whisk together flour, baking soda, and fine sea salt.
Combine: With mixer on low, gradually add dry ingredients to wet. Mix just until combined, do not overmix.
Add chocolate: Fold in chopped semi-sweet chocolate with a rubber spatula until evenly distributed.
Scoop: Scoop dough into 3-tablespoon portions (12 cookies). Place on prepared sheets, 3 inches apart.
Bake: Bake for 11-13 minutes until edges are golden and set, centers still soft.
Add toppings: Immediately out of oven, gently press one Malteser Bunny and one Mini Egg into each cookie.
Cool on sheet: Let cookies cool on baking sheet for 20 minutes.
Transfer and serve: Carefully transfer to wire rack. Enjoy warm or at room temperature.