Heat your oven to 180°C or 350°F and grease a 20cm baking dish with butter and line it with greaseproof paper. (note 1)
Melt the butter and chocolate in the microwave or in a Pyrex bowl over a saucepan of simmering water.
Mix in the beaten egg, sugar and vanilla essence. Then stir in the flour.
Pour into the baking dish and bake for 15-20 minutes in the middle of your oven until the top is set.
Make the Shredded Wheat nest topping while the brownie base bakes. Melt the chocolate, butter and golden syrup in the microwave or in a Pyrex bowl over a pan of simmering water.
Stir the crushed Shredded Wheat into the melted topping ingredients.
Remove the brownie base from the oven and check its doneness with a toothpick or similar implement. (note 2)
Add the Cream Eggs to the top of the brownie base and fill the space between them with marshmallows. Break the marshmallows into pieces to fill the smaller gaps if necessary.
Return the baking dish to the oven for 5 more minutes until the Cream Eggs and marshmallows are melted and merged.
Spoon the Easter nest topping mixture onto the top of the melted mid-layer.
Press the Mini Eggs into the Easter nest topping and leave the brownie to cool.
Chop into squares and enjoy when the Easter brownie has set.