Prepare pans: Preheat oven to 350°F (175°C). Grease and line three 6-inch round cake pans with parchment paper.

Mix wet ingredients: In a large bowl, whisk melted butter, granulated sugar, and brown sugar until smooth.

Add eggs and vanilla: Beat in the eggs one at a time, then stir in vanilla extract.

Combine dry ingredients: In a separate bowl, sift cocoa powder, flour, and salt. Gently fold into wet mixture until combined.

Bake layers: Divide batter evenly among pans. Bake for about 25 minutes, until tops are set and a toothpick comes out with moist crumbs.

Cool layers: Let cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Prepare frosting: Beat room-temperature butter and cream cheese until fluffy. Slowly add powdered sugar. Mix in vanilla bean paste, salt, and heavy cream until smooth.

Make chocolate frosting: Take a portion of frosting, add cocoa powder with reserved cream, and whip until blended.

Assemble cake: Place one brownie layer on a plate, spread vanilla frosting. Add second layer, spread chocolate frosting. Place third layer, cover with remaining frosting.

Decorate: Top with extra brownie chunks, cocoa powder, or chocolate shavings.