Heat your oven to 150°C Fan/170°C/325°F/Gas Mark 3.
Place your large baking tray into your oven to heat.
Grease your baking tin, then cut out two strips of greaseproof (parchment) paper which are the width of the tin and long enough to cover the base, both sides and allow for an overhang of about 2 inches on each side. You will hold the overhang later, to lift the cake from the tin (see picture in blog post).
Add boiling water to your saucepan, it needs to be about a third full. Keep to a gentle simmer on a low setting and place your heatproof bowl on top. Be careful, it will be hot. The base of the bowl must not touch the water below.
Add your butter to your heatproof bowl and gently melt.
Mix in your dark chocolate and granulated sugar and stir until melted.
Carefully remove your heatproof bowl, pour the mixture into your large bowl and leave to cool until at room temperature (needs to be cool enough not to cook the eggs when added).
Beat your eggs and then beat them, along with your vanilla extract, into your brownie batter with your balloon whisk, making sure they are totally mixed in.
Sift your flour, cocoa, baking powder and salt together and then beat into your brownie batter using your balloon whisk. Ensure everything is totally incorporated and no lumps of flour remain.
Pour into your prepared tin and level with the back of a spoon.
Place chunks of chocolate over the top of your brownie batter.
Place on your preheated baking tray in the centre of your oven and bake for 20 minutes, then remove and dot over your mini eggs. Push them down very slighly to secure.
Bake or another 10-15 minutes.
The brownies are ready when there is a slight wobble in the middle section. A skewer inserted into the centre should have some fudgy cake stuck to it but not lots of wet cake batter, if that is the case then bake for another 5 minutes and then test again.
Leave to cool completely in your brownie tin, then remove by lifting out holding the overhang of greaseproof paper.
Cut into 16 squares.