To make the Brownie Layer:
Place the butter, sugar, cocoa powder, and salt in a large saucepan set over medium-low heat. Warm the mixture, stirring frequently, until the butter melts, the sugar and cocoa powder dissolve, and the mixture is fairly hot. It will be grainy.
Add the vanilla extract and mint extract. Add the eggs one at a time, whisking well after each addition. As you add the eggs the mixture will go from being grainy and separated to a shiny, smooth, satiny texture. Finally, add the flour and fold it in gently until no streaks of flour remain.
To Make the Mint Chocolate Chip Mousse Layer:
Combine the white chocolate chips, 3/4 cup of cream, and salt in a microwave-safe bowl. Microwave in 30-second increments, whisking every 30 seconds, until the chocolate has melted and the mixture is smooth.
To Make the Chocolate Glaze:
Store Mint Chip Mousse Brownies in an airtight container in the refrigerator. They can be served chilled or left to sit at room temperature for about 15 minutes before serving.
For the Brownie Layer:
For the Mint Chocolate Chip Mousse Layer:
For the Chocolate Glaze: