Preheat the oven to 350°F. Line two baking sheets with parchment.: You will notice a faint warmth in the oven as it comes up to temperature, which ensures even browning; preheating prevents underbaked centers and inconsistent spread. The parchment creates a thin barrier so bottoms do not darken too quickly, and it makes cleanup painless. A common pitfall is skipping parchment or an uncalibrated oven, which can result in unevenly browned cookies. If your oven runs hot, set the rack to the middle to avoid overly dark bottoms.
In a large bowl or the bowl of a stand mixer, beat together 3/4 cup butter, 3/4 cup brown sugar, and 1/4 cup white sugar. Add 1 egg and 1 teaspoon vanilla and mix to combine.: At this stage the mixture should smell sweet and buttery, with the brown sugar lending a caramel note; creaming introduces air that helps the cookie lift. The texture will look creamy and slightly glossy when ready. Overbeating can introduce too much air, causing collapse while underbeating leaves sugar unincorporated, so mix until just smooth. If the butter was too cold you will see lumps, and if too warm it will appear runny, both of which affect final texture.
Mix in 2 cups flour, 2 teaspoons cornstarch, 1 teaspoon baking soda, and 3/4 teaspoon salt until barely combined (there should be some flour streaks still). This should be a thicker dough. Add in an extra Tablespoon of flour if needed. Mix in the 1 cup crushed mini cadbury eggs and 3/4 cup mini chocolate chips, just until combined.: The dough will transition from glossy to more matte and slightly stiff, holding its shape when scooped; you should feel a pleasant resistance. Leaving a few flour streaks prevents overworked gluten and yields tender cookies. Adding the candy and chips will create pockets of texture and tiny bursts of sweetness; stir gently to distribute without crushing the candies too much. A common error is overmixing here, which activates gluten and yields tough cookies, or under-adding flour resulting in a dough that spreads too much.
Use a medium cookie scoop to scoop the dough into 1-inch balls (golf ball size about 1.5 ounces). For a bigger cookie you could do 2-3 ounce balls. Place on prepared baking sheets. Bake 8 to 10 minutes, or until golden around the edges.: As the cookies bake you will hear a quiet soft bubbling and smell warm sugar caramelizing; watch the edges for a delicate golden tint while the centers remain pale. Smaller scoops yield more cookies and a higher ratio of crisp edge to tender middle, while larger scoops produce a bakery style cookie with a gooey center. A typical mistake is leaving them in too long which dries the centers, so aim for edge color not full top browning. Rotate the pans halfway through if your oven has hot spots.
Remove from oven and let cool on baking sheet before removing to a cooling rack. Store in a covered container up to 3 days.: Cooling on the sheet allows residual heat to finish setting the cookies while preserving a soft center; you may hear a faint settling sound as steam escapes and the cookies firm. Transferring too soon can cause cookies to break, while leaving them too long on the hot sheet can overcook the bottoms. For storage, an airtight container keeps surfaces tender; stacking with parchment between layers prevents sticking.
