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1
Preheat oven to 350u00b0 F.
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2
In the bowl of a stand mixer, cream together the butter and sugars until very light and fluffy. Add the eggs, one at a time, beating until well-incorporated after each addition. Add the vanilla and mix well.
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3
In a separate bowl, whisk together the flour, cocoa, baking soda, baking powder, espresso powder (if using) and salt. Add the dry ingredients to the wet ingredients by alternating with the buttermilk until you've added everything. Mix well.
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4
Line two jelly roll pans or rimmed half-sheet pans (make sure they have a decently high rim!) and VERY thoroughly butter or grease the foil. Pour a thin layer of cake batter on each pan (you should be able to use up half the batter on each, depending on the height of the rim of your pans. The batter shouldn't be deeper than 1/2-inch).
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5
Bake the cake for about 10 to 15 minutes, or until it just springs back when pressed lightly on the surface. Let cool in the pans for at least 15 minutes.
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6
Carefully remove the cake from the pans (inverting it onto a cutting board helps), and peel the foil off the back. Let it cool fully.
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7
While the cake cools, take out the ice cream to soften.
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8
Slice each sheet of cake in half. Spread a 1-inch thick layer of softened vanilla ice cream on each half-sheet of cake, then top with the second layer. You should have two large cake sandwiches. Wrap both tightly in plastic wrap and freeze fully (at least 2 hours).
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9
When ready to serve, remove from the freezer and slice the sandwiches into rectangles, about 5 by 2 inches. If you want to save some, you can wrap the individual sandwiches in plastic wrap for up to 1 month.
