Preheat oven to 350 F. Move the rack to the middle position.: You will notice a warm, neutral scent as the oven comes to temperature, and moving the rack to the middle ensures even heat circulation so the bars bake uniformly. This step matters because an off center rack can lead to uneven browning, with edges cooking faster than the middle. A common mistake is skipping the preheat, which causes longer bake times and uneven texture.
Melt the butter in a large, microwave-safe bowl. Let it cool for a few minutes while you prepare/measure out the rest of the ingredients and line an 8 x 8 baking dish with tin foil (and/or spray it with non-stick spray).: As the butter melts you’ll see a glossy, golden pool that smells rich and nutty. Letting it cool slightly prevents the egg from cooking when combined. Lining the pan makes removing the bars simple and protects edges from sticking. Avoid pouring piping hot butter onto the egg , which can produce scrambled bits.
Add the egg, vanilla, and sugar to the bowl and stir until it's combined, then stir in the baking powder and flour until smooth. Try not to over stir.: When you stir, the mixture will become glossy and slightly thick, releasing a warm sweet aroma from the brown sugar . Adding the dry ingredients gradually and folding them in maintains tenderness; over mixing develops gluten in the all purpose flour and makes the bars tough. Watch for streaks to disappear rather than stirring aggressively.
Stir in the white chocolate chips and Mini Eggs. Transfer batter to the baking dish and smooth it out.: The batter will feel slightly sticky and supple, and when you fold in the white chocolate chips and Cadbury Mini Eggs you will see colorful flecks throughout. Smoothing the top with a spatula ensures even thickness so the bars bake uniformly. A frequent issue is pressing the candy too deep, which can cause them to bleed color or sink, so fold gently and distribute evenly.
Bake for 30 minutes, or until a toothpick comes out (mostly) clean. You don't want gobs of raw batter stuck on it, basically. Let cool in the baking dish for at least 30 minutes. Cut into squares and enjoy.: As the pan bakes, the kitchen fills with a warm, sweet aroma, and the edges will set before the center, taking on a light golden hue. The toothpick test helps ensure the center is just set while remaining tender. Cooling allows the bars to firm up and makes cutting cleaner, otherwise you risk squashed squares. Don’t wait until the top is deeply brown, which signals overbaking and a drier bar.