Place your donut moulds on a tray and spray generously with cake release spray, making sure to reach the central column of each round.
Put the dark chocolate and butter in a large microwave safe bowl. Microwave in 30 second bursts, stirring between each burst, until melted. Avoid overcooking as this can negatively affect the chocolate.
Add the caster sugar. Stir through with a manual whisk - I say stir as you don’t want to add air but a whisk is still the best tool as it distributes everything quickly.
Add the eggs and vanilla. Stir with the whisk again.
Put the flour through a sieve into the bowl. Stir to combine.
Fill the Donut Moulds
Share the bronut batter evenly between the donut moulds. The batter should reach about 1cm (1/4 inch) shy of the top. I find it easiest to transfer the batter to a piping bag, snip off the end and pipe the filling in.
Scatter the chocolate chunks on top.
Bake
Place in the oven and bake for approximately 20 minutes, or until skewer comes out with wet, sticky crumbs. Timings will depend on the size and depth of your donut mould, whether your oven has a fan, how well it circulates heat and more so keep an eye on them towards the end of the baking time.
Transfer the moulds to a cooling rack and allow to cool to room temp, then flip the moulds over to release the brownie donuts.
If you brownies feel a bit soft or fragile, you can pop them in the fridge for 15 minutes to firm up.
Decorate Your Brownie Donuts
Sieve the cocoa and icing sugar into a bowl.
Add the vanilla and a little bit of the milk.
Whisk while gradually adding the milk until you have a thick icing. You might not need all the milk. Likewise, if you feel your icing is too thick, you can add a touch more milk.
Dip the donuts into the icing, flat side up and sit them back on the wire rack. You may like to place a tray or piece of baking paper beneath the rack to catch any drips.
Straight away, scatter the sprinkles over the icing.
Leave to set and then enjoy. The icing doesn't have to be completely set - I quite like it a bit sticky to match the squidgy brownie!