Preheat the oven to 170°C (340°F). Grease and line an 8-inch square tin with baking parchment.
In a large bowl, whisk together melted butter, sugar, and cocoa powder until smooth and glossy.
Add the eggs one at a time, whisking well after each addition. Stir in vanilla extract, vinegar, and red food colouring.
Fold in the flour and salt with a spatula until just combined. The batter should be thick and smooth.
Pour into the tin and spread evenly. Bake for 25-30 minutes or until the edges are set but the middle is slightly soft. A few moist crumbs on a skewer are perfect.
Cool completely in the tin before glazing.
To make the glaze, beat cream cheese and butter until smooth, then add icing sugar, vanilla, and enough milk or water to reach a drizzling consistency.
Drizzle over cooled brownies, then slice into neat squares once the glaze has set slightly.