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1
Preheat oven to 350F. Line a 11x17 half sheet pan with parchment paper and grease the sides with butter.
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2
In the mixing bowl of a stand mixer, rub orange zest into granulated sugar until fragrant and crumbly. Add flour, baking powder, baking soda, and salt. Mix on low speed with the paddle attachment for 30 seconds.
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3
Add butter and a quarter of the buttermilk into the mixer. Mix on medium speed for 3 minutes to aerate the batter and build some structure.
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4
In a small mixing bowl, stir together the remaining buttermilk and eggs. Add one third to the mixer and mix for 20 seconds to incorporate. Repeat two more times for the rest of the buttermilk mixture. The batter should be smooth and pourable.
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5
Fill the prepared pan with batter and level with offset spatula. Bake in the middle of the oven for 30 minutes or until the top is golden and cake tester comes out clean. Cool the cake overnight.
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6
When you're ready to assemble the cake, start with making the chocolate icing. In a large mixing bowl, sift together icing sugar and cocoa powder. Warm milk and butter in microwave until tepid. Whisk into icing sugar until smooth and homogenous. Whisk in Grand Marnier. Set the mixing bowl of glaze over a pot of simmering water at very low heat to keep the glaze fluid.
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7
Unmold the cake from pan and peel away the parchment paper. Cut the cake in half so you have two pieces each measuring 11x8.5. Spread marmalade over one piece of the cake in a thin layer using an offset spatula. Stack the two pieces together to make a big cake sandwich.
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8
Using a long serrated knife, trim the edges of the cake. Cut into 20 to 24 cubes. The yield depends on how much of the edge you trim away. The key is to match the size of the squares to the thickness of the cake sandwich so you end up with cubes.
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9
To set up your enrobing station, fill a medium mixing bowl with coconut. Place it close to the pot with the chocolate glaze (i.e. the stove). Set a cooling rack over a half sheet pan. It will be used for drying the enrobed cakes.
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10
Take one cube of cake sandwich and drop it gently into the bowl of chocolate glaze. Spoon glaze over the cake until it's all covered. Fish it out using a chocolate dipping fork or a regular dinner fork and let the excess glaze drip away. Drop the cake into the bowl of coconut and roll it around until all sides are covered with coconut. Use a clean fork to move the cake onto the cooling rack. Repeat until all cubes are enrobed.
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11
The glaze will set in about 30 minutes. Store in airtight container for up to 3 days.
