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1
Preheat the oven to 350 degrees F. Coat two 12-by-17-inch rimmed baking sheets with cooking spray, then line with parchment paper, leaving an overhang on the short sides.
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2
Prepare the cake mix with an extra egg.
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3
Divide the batter between the 2 baking sheets; spread evenly with an offset spatula.
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4
Bake until set, 8 to 9 minutes; let cool.
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5
Measure 2 1/2 inches in from each long side of one cake; mark with toothpicks.
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6
Score using a ruler, then remove the toothpicks and trim along the scored lines so the cake is 7 inches wide.
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7
Round the corners of the cake with a knife.
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8
Punch out holes with a straw (4 holes across, 10 down).
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9
Trim the other cake (no need to punch holes).
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10
Freeze the cakes 30 minutes.
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11
Invert the cake without the holes onto a clean baking sheet; peel off the parchment.
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12
Snip open 1 ice cream carton with kitchen shears, then slide the ice cream onto the cake.
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13
Repeat with the other 2 cartons of ice cream, arranging them side by side on the cake.
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14
Cover with plastic wrap and press with your hands to smooth the ice cream and fill the gaps.
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15
Remove the other cake from the parchment with a thin spatula; place it on the ice cream.
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16
Trim any excess cake and smooth the sides.
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17
Freeze at least 2 hours before serving.
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18
Photograph by Andrew Purcell
