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1
For the cakes: Preheat oven to 350 degrees.
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2
In a mixing bowl, cream together the butter and brown sugar.
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3
Add the egg and vanilla extract and beat until light and creamy.
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4
In a separate bowl, whisk together the baking soda, salt, flour and cocoa.
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5
Add dry ingredients to butter mixture in three parts, alternating with buttermilk, and combining well after each addition.
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6
Using an ice cream scoop or a spoon, scoop out 12 1/4-cup mounds of batter and place about 6 inches apart on a parchment-lined baking sheet.
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7
Bake until tops are puffed and cakes spring back when touched, 12 to 14 minutes.
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8
Remove from oven and cool completely before filling.
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9
For the buttercream filling: For best results, follow directions carefully, paying attention to required temperatures.
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10
Fill bottom half of a double boiler (or a medium saucepan) with an inch or two of water, and bring to a boil over high heat.
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11
In top half of double boiler (or a metal bowl), combine egg whites and sugar.
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12
Place over simmering water and whisk just until sugar is dissolved and temperature reaches 180 degrees on an instant-read thermometer.
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13
Using a whisk attachment on a heavy-duty mixer, whisk egg whites and sugar on high until they double in volume and become thick and shiny.
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14
Continue to whisk until cool.
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15
Reduce speed to medium and begin to add butter about 1/2 tablespoon at a time, until all the butter is incorporated.
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16
Add vanilla and salt.
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17
If mixture looks curdled, continue to whisk until it is smooth.
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18
Increase speed to high and whisk for 1 more minute.
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19
Use immediately or place in an airtight container and chill for up to 3 days, whisking buttercream again before using.
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20
For assembly: Using an ice cream scoop or spoon, place 1/4 cup buttercream on flat side of each of 6 cakes, spreading it to edges.
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21
Top filled half with another cake to sandwich the buttercream.
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22
Store in an airtight container at room temperature for up to 3 days, or wrap individually and freeze for up to 3 months.
