Break up the 185g/6.5oz milk/semi-sweet chocolate & microwave in 30 second bursts, stirring well in-between each, until nearly melted. Then add in the cubed butter & mix well & then continue heating & mixing till all melted. Leave to cool while you gather the other ingredients. Alternatively, melt in a bowl over a pan of barely simmering water.
Weigh out the flour & cocoa through a sieve into another bowl (not the main mixing bowl).
Grease the baking tin & place baking paper/parchment inside it. See photo 1 above. If you want to learn how to cut your paper to fit a square tin and not run the risk of the paper falling into the brownies while baking, read my Lining Cake Tins Lesson. I usually hold the paper in position with Frylight baking/cooking spray (see Photo 1 above), or use my home made Cake Lining Paste Next whisk the eggs & sugar with a hand/stand mixer for approximately 4 – 5 mins, on medium to high speed. It must become thick & creamy, paler in colour & increased in volume. This is the key to these gooey brownies. See Photo 2 above.
Gently fold in the melted chocolate/butter with a plastic/silicone baking spatula & do so in the shape of a number 8. Mixing too vigorously will undo all the whisking from earlier. Mix just until all mixed in and no more. See photo 3 above.
Next re-sieve the flour & cocoa mixture, into the batter. Mix it in again like before until no flour is visible, and scraping down the sides too. See Photo 4 above.
Roughly chop the remaining 100g/3.5 oz dark/bitter chocolate & finally fold into the batter just till evenly distributed. See Photo 5 below.
Pour the brownie batter into the prepared pan & level off if required. See Photo 5 above. Leave to sit so that it will form a crust later. (Leave at least 10 mins). Heat up the oven to: 180°c/160°c Fan oven/350°f/Gas mark 4 while the brownie sits.
Bake for about 18 – 25 minutes, turning the tray if needed, after half-way through. The top will have formed the crust, (see photo 6 below). Testing with a wooden cocktail stick, & the outer edges produce a clean stick whist the centre is a little moist with just a few crumbs. This is what we want.
Immediately on removing from the oven, you need to add your mini chocolate eggs. You use whole ones (depending on the size), or cut in half. Some are easier to cut in half than others. I used Cadbury's Mini Eggs and chopped some of them. I also used Hershey's white chocolate eggs. Since I cut the brownies into 16 squares, I wanted the eggs to be in the centre of each brownie square. And so positioned the eggs in a pattern, as seen in photo 7 below, and then added on some cut eggs too. For the hard shelled Mini Eggs, you can push down slightly into the brownie. However, if the egg is just all chocolate, it will start to melt with the residual heat form the brownies. So don't push those ones in.
Leave the brownies to sit to cool down in the tin for 5 minutes. Then transfer via the paper, to the cooling rack to cool down completely before cutting into 16 squares. See Photo 8 below. See also tips in the notes for cooling quicker & cutting easier.
Store left-overs in an airtight container for a few days.