Melt the butter either in the microwave or over a bain-Marie.
Beat the sugar and salt into the butter with a handheld balloon whisk.
Add the egg, yolk and vanilla extract & beat in.
Mix together the plain flour, bread flour and bicarbonate of soda. Stop beating when no streaks of flour remain - do not overbeat it.
Put 150g of the Mini Eggs into a bag and smash them roughly using a rolling pin or a pestle and toss them into the bowl. Don't break them up too small, large chunks are better in the dough.
Fold through the broken Mini Eggs using a large metal spoon.
Cover and chill the dough for at least 6 hours.
Preheat the oven to 180C/ 350F/ GM 4 and line several baking sheets with baking parchment.
Divide the dough into 14 balls (approximately 50g each or about the size of a ping-pong ball). I use digital kitchen scales to ensure my cookies are evenly sized.
Place on baking sheets (at least 10cm apart – bake in batches as necessary). I bake mine in 2 batches as I only put 4 cookies onto each baking sheet.
Bake for 10-12 minutes, turning the baking sheets halfway through. (Baking for 10 minutes lets the centre of the cookies remain gooey, while cooking for 12 minutes turns them less gooey and more chewy.)
From the remaining Mini Eggs, reserve 1 per cookie and break the rest in half.
Remove the cookies from the oven and firmly tap the baking sheet on a heatproof worktop to help the cookies deflate.
Optional: reshape the cookies using a large cookie cutter and the hula hoop technique: place a cookie cutter that is a little larger than your cookie over each one and firmly shift it around, forcing the cookie to move around and bang into the cookie cutter. It helps push the cookie into a circular shape. This must be done as soon as the cookies are taken out of the oven.
While the cookies are still hot, press 1 whole Mini Egg into each cookie and 5-6 chunks of Mini Egg around it.
Let rest for 2 minutes on the baking sheet then transfer to a wire cooling rack.
Cook the second batch of cookies from step 11.