Heat oven to 350°F. Line 13-inch by 9-inch pan with foil; grease foil.
In a 3-quart saucepan, melt butter and chocolates on low, stirring. Remove from heat.
In a bowl, whisk flour, cocoa, and salt. In a large bowl, with an electric mixer on high speed, beat eggs until blended. Gradually add sugars; beat until tripled in volume, about 10 minutes.
Fold in chocolate mixture and vanilla, then flour mixture. Pour into pan. Bake until a toothpick inserted near center comes out almost clean, 28 to 32 minutes.
Once cool, cut brownies into hearts freehand or using a heart-shaped cookie cutter. (Tip: Don't let the trimmings go to waste — you can even crumble them onto a sundae!)
Pipe on store-bought frosting to create cute chocolate "conversation hearts."
