Mint Chip Ice Cream Cake

Preparation

1. For cake:

   Step 1

   Preheat oven to 350°F. Coat a large rimmed baking sheet (17x13x1") with nonstick spray. Line bottom with a large sheet of parchment paper.

   Step 2

   Whisk flour, baking powder, and salt in a medium bowl to blend; set aside. Using an electric mixer, beat egg whites on medium-high speed in a large bowl until light and frothy, 1-2 minutes. Increase speed to high; add cream of tartar. With mixer running, gradually add sugar. Beat until meringue is soft and pillowy, is tripled in volume, and medium peaks form, 8-10 minutes.

   Step 3

   Using a spatula, gently fold dry ingredients into meringue, being careful not to deflate. Once flour is almost incorporated, fold in butter, oil, and vanilla; continue folding until no dry clumps of flour remain. Pour batter into prepared sheet; smooth top.

   Step 4

   Bake until cake is light golden brown and springs back slightly when pressed, 16-19 minutes. Transfer to a wire rack and let cool in pan for 15 minutes. Invert cake onto rack; peel off paper and let cool completely.

   Step 5

   Using a serrated knife, halve cake crosswise into 2 smaller rectangles. Cover and freeze at least 1 hour or up to 5 days ahead.

2. For filling and assembly:

   Step 6

   Place 1 cake rectangle on a large platter or cake stand. Spread 1/2 cup ice cream evenly over cake. Sprinkle crushed cookies evenly over. Spread remaining ice cream over, being careful to keep cookie layer evenly intact. Top with remaining cake rectangle, pressing slightly to adhere. Using an offset spatula, smooth sides of cake. Freeze until ice cream is firm, about 2 hours.

   Step 7

   Place 2 tablespoons cold water into a small heatproof glass or metal bowl. Sprinkle gelatin over; let soften, about 10 minutes.

   Step 8

   Pour water to a depth of 1/2" into a small skillet set over medium heat. Transfer bowl with gelatin to skillet; stir until gelatin dissolves, about 2 minutes. Remove bowl from skillet.

   Step 9

   Place chilled cream in a large bowl. Using an electric mixer, beat cream while gradually adding melted gelatin. Beat until soft peaks form. Add powdered sugar and continue to beat until medium peaks form and whipped cream is soft and pillowy (not stiff or grainy). Add green food coloring to achieve a light mint-green hue, 8-10 drops; fold to blend.

   Step 10

   Using an offset spatula, spread whipped cream over entire cake, creating dramatic swirls on top. Scatter sprinkles over. Freeze until cream is set, at least 1 hour. DO AHEAD: Cake can be made 2 days ahead. Keep frozen.