In a large bowl, using an electric/stand mixer (or hand whisk, just double/triple the times) combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg and vanilla and mix again.
Add the flour and baking powder, plus a pinch of salt, and fold in using a rubber spatula or wooden spoon, until just combined. The dough should be fairly solid at this stage (it ought to almost form a ball, albeit a very soft one!). Add the chocolate and crushed Ferrero Rochers (keep a few back for the end) and stir together.
Let this dough chill in the fridge for at least 30 minutes.
Preheat oven to 200C/180C Fan/Gas Mark 6. Line 2-3 large baking trays.
Scoop tsp sized balls of cookie dough – so you have 16 of similar size. Place on the trays, with about 1-2 inches in between (max 4 per tray).
Bake for 9-11 minutes, until just golden at the edges. They may look underbaked in the centre but they keep cooking from the residual heat, and this gives them their perfect soft texture. Add some of the remaining Ferrero Rocher now - they'll melt just the right amount.
Repeat for all cookies and leave to cool on a wire rack before serving.