Preheat oven to 325°F. Line an 8x8 baking pan with parchment.

Combine the butter, sugar, cocoa powder, and salt in a medium bowl. Set the bowl over a pan of simmering water to make a double boiler. Stir often until the butter is melted and the mixture is well combined. Let cool until the mixture is warm, about 15 minutes.

Stir in the vanilla. Add the eggs one at a time, beating each one into the mixture well. When the mixture is thick and shiny. Stir in the flour just until you can no longer see it.

Bake 20 to 25 minutes, or until a toothpick inserted 1 inch from the edges comes out mostly clean. Cool completely before frosting.

To make the frosting, beat together all of the ingredients in a large bowl until light and fluffy. Add more cream if desired to thin the frosting out a bit or leave it thick and fudgy.

Spread the frosting over the cooled browneis and cut into 16 squares.