Preheat the oven to 180°C/160°C Fan. Line a couple of baking sheets with non-stick baking paper and set aside.
In a large mixing bowl, cream together the softened butter with the soft light brown sugar and golden caster sugar until creamy. It is best to do this in a stand mixer fitted with a paddle attachment, but you can use an electric whisk or a wooden spoon and good old-fashioned elbow grease!
Add the egg and vanilla extract and beat again until combined.
Sift in the plain flour, baking powder, bicarbonate of soda and salt. Mix until just combined to form a smooth dough - take care not to overmix or your cookies will be tough!
Add the chopped chocolate and crushed mini eggs and fold through until evenly distributed throughout.
Using an ice cream scoop, scoop the cookie dough into around 9 equal-sized balls. Use your hands to roll into proper balls and place roughly 5 cm apart on the prepared baking sheets (I like to do 5 on one sheet, 4 on the other so they have plenty of space to expand). At this point, if you have the time, it is well worth popping your cookie dough into the fridge to chill. Anything from 1 hour to overnight will improve the texture of your cookies. However, I know this doesn’t work for everyone, so if you don’t have the time, simply continue with the next step - the cookies will still taste incredible! You can also cheat and blast them in the freezer for 30 minutes.
Bake for about 12 minutes - or until the edges are just golden-brown but the centres are still puffy and slightly gooey. They might look underdone, but they will continue to bake as they cool on the trays.
Leave to cool a little before eating so they have time to firm up. Obviously they are delicious warm from the oven - but try to give them at least 15 minutes so you get the perfect chewy texture of the cookies.
To serve (optional): drizzle with a little melted white chocolate and sprinkle with extra whole or crushed mini eggs.