1½ cups plus 6 tablespoons unsalted butter
30 ounces 60% bittersweet chocolate, coarsely chopped
¾ cup heavy whipping cream
6 tablespoons Lyle's Golden Syrup (or substitute honey)
2 7½-ounce packages lightly toasted butter biscuits (like Le Petit Beurre), coarsely chopped
8 ounces semisweet chocolate, coarsely chopped
1 cup heavy whipping cream
1 tablespoon light corn syrup
1. Lightly coat a 9-inch springform pan with nonstick spray. Line bottom of pan with parchment paper. Stir the butter and chocolate together in a large saucepan over low heat until melted and smooth. Stir in the cream and golden syrup (or honey). Remove from the heat and mix in the chopped biscuit pieces to evenly incorporate. Pour the chocolate mixture into prepared pan; smooth top. Cover and refrigerate until set, at least 3 hours or overnight.
2. To make the glaze, put the chopped chocolate in a medium bowl. Bring the cream and corn syrup to a simmer in a small saucepan over medium heat. Pour over the chocolate. Let stand for 5 minutes, then stir until the chocolate is melted and smooth. Let cool slightly, stirring occasionally, until the glaze is slightly thickened but still pourable.
3. Wrap a warm, damp kitchen towel around the cake pan. Remove the pan sides. Invert the cake onto a wire rack set on a wax paper-lined baking sheet. Remove the cake pan bottom and peel off the parchment paper. Pour glaze over the cake, allowing it to drip down the sides; spread as needed to cover the cake. Refrigerate until the glaze sets, about 30 minutes.