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1
The night before: Separate eggs and let egg whites sit on the counter overnight, covered with a clean cloth. Pour cream into a saucepan. Split and scrape vanilla bean into the cream and heat until just boiling. Remove from heat and let cool. Cover and chill overnight.
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2
The next day: Preheat oven to 350u00b0 F. Prepare a 9 x 12-inch cake pan with buttered parchment. (Alternatively, you can prepare two 9-inch round cake pans.)
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3
In the bowl of a stand mixer fitted with a paddle attachment, cream butter and brown sugar until creamy and fluffy, about 3 to 4 minutes.
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4
Whisk together flour, baking powder, espresso powder, and salt. Add mixture to butter and sugar in 3 increments, mixing fully between additions.
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5
Whisk together egg yolks and milk. Add to batter in 3 increments, incorporating fully each time. Scrape batter into prepared pan.
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6
In a clean mixing bowl (I wipe down my bowl and whisk attachment with vinegar), whip egg whites until foamy. Add sugar and cinnamon and whip to soft peaks.
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7
Spread the meringue over cake batter, cover with toasted walnuts and bake 35-40 minutes, until the meringue is crackly. Let cake cool completely.
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8
Meanwhile, remove vanilla bean from cream. Add maple syrup and whip cream to soft peaks. Cut cake in half and place one half on a cake plate, meringue side up. Cover with whipped cream. Top with remaining half, meringue side up. You will end up with a 2-layer square cake from your single-layer rectangular cake. (If you're doing this in two round cake pans, just top one with the other.) Let sit 1-2 hours.
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9
To go completely over-the-top, serve with a dark chocolate sauce punched up with a few dashes of walnut bitters.
