Preheat the oven to 350°F. Line two baking sheets with parchment.: The kitchen fills with a warm, anticipatory scent as the oven comes up to temperature, which helps create steady heat that produces even browning on the cookie edges. You should hear the faint hum of the oven and see the racks settle; this consistent environment prevents underbaked centers. A common mistake is skipping parchment, which can cause sticking or uneven bottoms, so always prepare the sheets first.
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 you will notice the sugars beginning to dissolve into the softened butter , creating a creamy, fluffy mixture with a rich, toffee like aroma. The sound of the mixer will change from a gritty buzz to a smooth churn as the fats and sugars emulsify, which traps air and contributes to a tender bite. If you overmix here you can incorporate too much air leading to cakier cookies, so stop once the mixture is light and homogenous.
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.: As the dry ingredients are folded in, the dough will transform from glossy to a thicker, slightly shaggy mass with visible streaks of flour . You will smell a more pronounced baked grain scent as the leavening mingles. The cornstarch helps maintain a soft center, and the minimal mixing keeps gluten development low for tenderness. Avoid overworking the dough, which can lead to tough cookies, and be careful adding the candy so it does not melt or bleed excessively into the dough.
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 settling and notice the edges beginning to set first, turning a light golden brown while centers remain slightly domed and pale. The aroma will be buttery and sweet with chocolate notes rising. These visual cues indicate the right moment to remove them so the centers remain soft. A common pitfall is baking too long; the cookies continue to firm as they cool, so pull them when the edges are golden and the center looks slightly underdone.
Remove from oven and let cool on baking sheet before removing to a cooling rack. Store in a covered container up to 3 days.: The cooling phase is critical, as the cookies finish setting on the warm sheet, often releasing a final puff and developing texture. You will feel the edges firm beneath your fingertips and may still find the center tender, which is the ideal balance. If you transfer them too early they can fall apart, and if you store them while completely hot you can create condensation and sogginess, so allow them to rest before stacking in a container.