Preheat your oven to 350°F (175°C) and line the baking pan with parchment paper for easy removal.
In a large mixing bowl, cream together the softened butter and brown sugar until the mixture is smooth and fluffy. This smells sweet and creamy, with a slight shine on the surface.
Beat in the eggs one at a time, mixing well after each addition until fully incorporated. The mixture will become more pale and slightly thickened.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually fold the dry ingredients into the wet mixture using a silicone spatula, mixing just until no streaks remain. The dough will be thick and lightly crumbly but hold together when pressed.
Spread the cookie dough evenly into the prepared baking pan, pressing it down to form a smooth, compact layer that reaches all sides.
Bake in the preheated oven for about 15-20 minutes, or until the edges are golden and the center looks set. You may see slight bubbling around the edges.
Remove the pan from the oven and immediately sprinkle the mini eggs and chocolate chips evenly over the warm surface. Gently press some of the mini eggs into the dough for better adherence.
Allow the bars to cool in the pan for about 10-15 minutes, until they firm up a bit and the chocolate begins to set.
Transfer the entire block of cookie bars by lifting out the parchment paper, then cut into squares or rectangles.
Serve the bars slightly warm or at room temperature, enjoying the combination of crispy edges, chewy center, and crunchy mini eggs on top.
