Pre-heat oven to 170 degrees celsius (fan-forced). Grease and line a 20cm X 20cm square baking tin with baking paper and set aside.
Place the chocolate and butter into a microwave-safe bowl and melt on 50% power for 2- 3 minutes or until just melted (stir every 30 seconds). Allow to cool slightly.
Add the eggs and whisk through.
Add the sugar, vanilla extract and Baileys and whisk through.
Sift over the plain flour, cocoa powder and bi-carbonate soda, add the salt and fold through the chocolate mixture.
Pour into the prepared tin and bake for 20 minutes or until a toothpick comes out slightly moist. Allow to cool for 20 minutes before frosting.
To make the frosting, whisk the melted butter, vanilla extract, pinch of salt, icing sugar (sifted), cocoa powder (sifted) and Baileys Irish cream together in a bowl until smooth and creamy.
Spread the frosting over the brownies, sprinkle over the sea salt flakes and allow to cool completely. Store in an airtight container at room temperature for up to 5 days.