Preheat oven to 335. Line a 13×9 cake pan with parchment paper.

Over a double boiler, melt the butter.

Once melted, add the cocoa and sugar and whisk to combine. Continue cooking until the heat registers 140 F.

Remove from heat and transfer to a bowl.

Once cool, add salt and vanilla and whisk.

Add eggs all at once and whisk until you have a smooth, shiny batter, about 40 seconds.

Add flour and whisk until just combined.

Pour batter into cake pan and bake for about 25 minutes, until the center is slightly puffed.

Let cool completely in pan. Slice in half into two equal squares.

In a bowl beat together the sugar, hazelnut butter or nutella, sweetened condensed milk, and salt (and cocoa if using).

Add the heavy whipping cream and beat to medium peaks.

Spread the filling on top of one half of the brownie layers.

Add the other layer on top, cover with plastic wrap and set in the freezer overnight.

Slice into squares or thin rectangles. Store in the freezer in an airtight container.