Preheat oven to 350 degrees F. Line the bottom of a 9-inch springform pan with parchment paper and lightly butter the sides. Combine butter and chocolate in a medium saucepan over low heat, stirring constantly until completely melted and smooth. Remove from heat and let cool 5 minutes.

Whisk sugar into the chocolate mixture until combined. Add eggs one at a time, whisking well after each, then stir in vanilla. Fold in flour and salt just until no streaks remain. Do not overmix.

Spread batter evenly in prepared pan. Bake 22-25 minutes until a toothpick inserted in center comes out with moist crumbs. Cool completely in pan on a wire rack, about 1 hour. The brownie will sink slightly as it cools - this is normal.

Puree 1 pound of strawberries with 1/4 cup sugar in a blender until completely smooth. Press through a fine-mesh sieve to remove seeds; you should have about 1.5 cups puree. Set aside.

Sprinkle gelatin over cold water in a small bowl. Let stand 5 minutes to soften. Microwave 10-15 seconds until liquid, then whisk until smooth. Stir immediately into the strawberry puree.

Beat heavy cream with remaining 1/4 cup sugar to stiff peaks. Gently fold in the strawberry puree in three additions, keeping the mixture as airy as possible. Fold in remaining 8 ounces strawberries, chopped small.

Pour mousse over cooled brownie base, spreading evenly. Refrigerate at least 3 hours or overnight until completely set. Run a thin knife around the edge before releasing the springform. Top with fresh strawberries just before serving.