Preheat your oven to 180℃ (or 160℃ if you're using a fan oven). Line two large trays with baking paper.
Add the butter to a large bowl with the caster sugar and brown sugar. Use an electric whisk or stand mixer to combine it into a thick paste.
Add the vanilla extract and the egg and mix again.
Next, add the flour. Mix well and when it starts to come together, use your hands to press it into a ball of dough. If it's too wet, add more flour and if it's too dry, add a splash of milk.
Add the Kinder chocolate segments and the chocolate chips, setting a few aside for topping the cookies if desired. Gently knead them into the cookie dough with your hands.
Divide the dough into 12 equal-sized pieces and roll each into a ball.
Place them onto the tray, leaving a couple of inches between them to allow space for spreading. I usually do 4-5 cookies per tray.
Bake the cookies, one tray at a time, for 11-12 minutes until they are JUST turning golden brown at the edges. They'll look a bit underbaked in the middle at first but will firm up as they cool.
While the cookies are still warm and gooey, gently press the Kinder Bueno pieces and the Kinder Hippos (if using) on top.
Leave the cookies on the tray until they're completely cool then enjoy!