Center a rack in the oven and preheat to 350 degrees F. Spray the inside of an 8-cavity 2" heart-shaped silicone pan with non-stick spray and place on a baking sheet.
Melt the chopped chocolate and butter in a microwave-safe bowl on low power for 30-second bursts. Stir after each burst.
Use an electric mixer to beat the egg and yolk in a bowl until frothy. Add the sugars and whip until very thick and pale colored, about 5 minutes. Add the melted chocolate mixture and blend completely.
Sift together the flour and cocoa powder and add the salt. Add to the mixture in 3 stages, blending well after each addition. Stir in the walnuts.
Divide batter evenly among cavities of the pan. Bake 35 minutes, until a tester comes out slightly moist. Remove from the oven and cool the pan on a rack. Turn the pan upside-down and gently peel away from the brownies, then turn brownies top-side up.
Store in a single layer in a tightly sealed container or covered with foil at room temperature up to 5 days.