In a large mixing bowl, add the room-temperature butter, granulated sugar, brown sugar, and salt. Cream with an electric hand mixer for 2–3 minutes, or until the mixture is light, fluffy, and pale in color.
Add the egg, egg yolk, and vanilla extract, and mix again until smooth and creamy.
Add the flour and baking soda, and mix on low speed until the dough is almost combined.
Fold in the chocolate chips and chopped Kinder pieces with a spatula until everything is evenly distributed and the dough is just combined.
Cover the bowl with plastic wrap and chill the dough for at least 1 hour. For even better flavor and texture, chill overnight.
Preheat the oven to 180°C (350°F) and line a baking sheet with parchment paper. Using a cookie scoop, portion the dough into 1½-tablespoon balls and place them on the prepared baking sheet.
Bake for 11–12 minutes, or until the edges are lightly golden and the centers look soft.
While the cookies are still warm, press a few extra Kinder pieces on top for a melted, bakery-style finish.
Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.
Notes
Use room-temperature butter and eggs. This helps the dough mix smoothly and creates a soft, even cookie texture.
Cream the butter and sugars well. Mixing until light and fluffy adds air to the dough, giving the cookies the perfect chewy texture.
Measure flour correctly. Too much flour makes cookies dry. Use a kitchen scale or spoon-and-level method for accuracy.
Chill the dough. Resting the dough for at least 1 hour prevents the cookies from spreading too much and deepens the flavor.
Use a cookie scoop. It keeps the cookies the same size so they bake evenly.
Fold in Kinder gently. Kinder chocolate is soft, so fold it in with a spatula to keep the chunks intact.
Don’t overbake. Remove the cookies when the edges turn slightly golden, and the centers still look soft. They’ll set as they cool.
Add Kinder on top after baking. Pressing extra chunks on top while the cookies are warm makes them extra pretty and melty.
Let cookies cool on the tray. A short cooling time helps them finish setting without overcooking.
Freeze leftover dough. Kinder cookie dough freezes beautifully, so you can bake fresh cookies anytime.