In a big mixing bowl, combine butter, light brown sugar, and caster sugar, then whisk until the mixture is light, creamy, and fluffy.
Combine an egg and vanilla extract in the mixing bowl and whisk until well blended.
Combine the plain flour, baking powder, and bicarbonate of soda in the mixing bowl and whisk until a cookie dough has formed.
Add the mini eggs and mix.
Divide the dough into 8 equal pieces and roll into a ball. Place on a plate or baking tray and let them cool in the freezer for 30 minutes to firm up.
Preheat the oven to 200℃/180℃FAN/Gas 6/400℉. Line a baking tray with baking paper.
Place the cookie balls on the baking tray and bake for 14 - 16 minutes. Leave on the tray for 10 minutes before transfer to a wire rack to completely cool.
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