Preheat oven to 350 F. Butter a 11X17 sheet pan. Line with parchment paper. Butter parchment paper. Dust pan with high quality unsweetened cocoa powder.
Melt butter and chocolate in a double boiler over simmering water, stirring until smooth. Remove from heat and cool to lukewarm.
Whisk sugar and eggs together in a large bowl. Stir in vanilla extract. Add chocolate and stir to combine well.
Pour batter into prepared pan. Rotate pan once during cooking. Bake until set and top begins to crack, about 25 -30 minutes. (The center will still be moist.) Cool pan completely on rack. Turn out on a sheet pan or cutting board. Chill again. Cut with large heart shaped cutter.
Transfer cake to serving plate. Dust with sifted powdered sugar. Sprinkle with pomegranate seeds.