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1
Move rack to middle of oven and preheat to 350u00b0F. Grease one 8 inch cake pan and line the bottom with a parchment circle.
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2
Combine flour, baking powder, salt, and cinnamon in a bowl and whisk for 30 seconds to combine completely. Set aside.
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3
Whip the egg whites until they are foamy. Add 1 teaspoon sugar and beat until they reach soft peaks. Cover and set aside.
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4
Add butter to the bowl of a stand mixer fitted with the paddle attachment. The butter should be cool-room temperature, meaning it should give when poked but not so soft that it barely holds its shape. Whip butter until it is light and airy, about 3 minutes. Add sugar and beat on medium speed for 5 minutes. Add yolks, one at a time, beating each for 30 seconds before adding another. Add in orange zest, beating for another 30 seconds.
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5
Reduce mixer speed to low. Add 1/3 of the dry ingredients, mixing just until combined. Add sour cream, mixing just until combined.
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6
Add another 1/3 of the dry ingredients, mixing just until combined, then add 1/3 cup condensed milk, reserving the rest, again mixing just until combined. Add the rest of the dry ingredients. Mix for 10 seconds or so, just until there are no traces of flour left. Gently fold in egg whites, making sure to not over mix the cake, as you want it light and airy.
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7
Scrape the batter into the prepared cake pan and smooth the top flat with a spatula. Cake for 45 minutes, or until a toothpick inserted in the middle comes out clean.
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8
Remove cake from oven and let sit in the pan for 10 minutes. Turn out onto a cooling rack and allow to cool completely. Once cool, set the cake on a serving plate and poke a few holes in the cake using a thin skewer, spacing the holes out over every 2-inches or so.
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9
In a small bowl, combine heavy cream with the remaining 1 cup of condensed milk and mix well. Pour 1/3 of the cream over the cake and let sit for 30 minutes. Follow with another 1/3 cup of cream, letting the cake sit for another 30 minutes, then pour over the remaining 1/3 cup of cream. Cover cake and refrigerator for 2 hours.
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10
Once cake is well chilled, frost with Swiss buttercream. If you like, decorate the top with a bit of chocolate ganache in a pretty pattern around the edges. Store in the refrigerator until ready to serve.
