Preheat oven to 350°F. Butter an 8-inch square metal pan, line with parchment paper and butter again. Set aside.
Make cake: In a small saucepan, melt chocolate, butter and crème fraîche over low heat, stirring until smooth. Let cool.
Using an electric mixer, with a whisk attachment if possible, beat eggs, sugar and vanilla until thick. Sift flour and baking powder into mixture and fold in. Fold in chocolate mixture. Pour batter into prepared pan and bake until a toothpick inserted in the center comes out clean, 40 to 45 minutes. Let cool briefly in pan, then turn out onto a rack to cool completely.
Make filling: In a small saucepan, melt crème fraîche and butter over medium heat; stir until smooth. Stir in chocolate until melted. Stir in confectioners' sugar and brandy. Let cool, then refrigerate until thick, about 30 minutes.
Split cake horizontally, using a long serrated knife. Transfer half to a platter; spread filling thickly. Top with other layer. Spread remaining filling on top and sides.
Make covering: In a bowl, melt chocolate in microwave oven, stirring. Stir in golden syrup. Transfer to a large plastic bag and refrigerate until firm enough to roll.
Decorate cake: Lightly dust a work surface with cocoa powder, then place chocolate covering on top. Dust chocolate covering with more cocoa and roll out with a rolling pin, lifting often to make sure covering does not stick. Lift and gather the covering, then drape it over cake, pressing gently against cake's sides. Trim away any excess at base with a sharp knife.
Lightly dust top of cake with cocoa powder. Dab golden syrup randomly over cake. Press gold-leaf against syrup, then lift off to leave small dots of gold on top of cake. Serve at room temperature.
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