Prep the pan and oven. Heat oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment.

Make the brownie batter. In a microwave-safe bowl, melt butter and 4 oz chocolate in 20-second bursts, stirring until smooth. Whisk in granulated sugar and brown sugar until glossy. Add eggs and vanilla; whisk until thick and shiny, about 30 seconds.

Finish the batter. Sift in flour, cocoa powder, and salt. Fold gently with a spatula just until no dry streaks remain. Do not overmix.

Bake the base. Spread batter in pan. Bake 18–22 minutes, until edges are set and a toothpick comes out with a few moist crumbs. Do not overbake. Cool completely in the pan on a rack.

Melt chocolate for silk layer. Melt 6 oz chocolate and let it cool to room temperature. It should be fluid, not hot.

Start the silk filling. In a mixer, beat softened butter on medium-high until creamy, 1 minute. Add powdered sugar, cocoa, and salt. Beat until fluffy, 2 minutes, scraping the bowl.

Stream in eggs safely. With mixer on medium, add pasteurized eggs one at a time, beating 1–2 minutes after each addition. The mixture will become lighter and silky.

Add chocolate and vanilla. Beat in cooled melted chocolate and vanilla until fully combined and smooth, about 1 minute. The filling should look glossy and airy.

Layer and chill. Spread the silk filling evenly over the cooled brownie base. Cover and chill at least 2 hours, or until set.

Whip the cream. In a cold bowl, beat heavy cream, powdered sugar, and vanilla to medium-soft peaks. Spread over the set silk layer.

Garnish and serve. Add chocolate curls, chips, or a light cocoa dusting. Slice with a hot, clean knife. Serve chilled.