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1
Heat the oven to 350 degrees F. Grease a 13 by 17-inch sheet pan and line the bottom and sides with parchment or waxed paper.
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2
Sift together the 3 cups sugar, flour, cocoa, baking powder, baking soda, and salt.
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3
Transfer to a mixer fitted with a whisk attachment (or use a hand mixer) and blend briefly.
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4
Whisk together the eggs, milk, oil, and vanilla extract in a medium bowl and add to the dry ingredients and mix at low speed for 5 minutes.
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5
Gradually add the hot water, mixing at low speed just until combined.
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6
The batter will be quite thin.
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7
Pour the batter into the pan.
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8
Bake until a tester inserted in the center of the cake comes out clean (a few crumbs are okay) and the center feels firm to the touch, 25 to 30 minutes.
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9
Let cool in the pan.
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10
Chill, covered, until ready to build the cake.
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11
Make room in your freezer for a serving platter that you'll build the cake on.
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12
Place a piece of parchment over the entire cake and flip it out onto another sheet pan.
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13
Peel off the original parchment and cut the cake in half so you have 2 pieces 13 by 8 1/2 inches.
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14
Place a piece of cake on a rectangular serving platter and schmear the ice cream on it all the way to the edges.
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15
Smooth it out with an offset spatula and place back in the freezer.
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16
After it has chilled 15 minutes or so place the second layer of cake on top, cover with parchment paper and press down with a sheet pan gently to stick the top layer.
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17
Return to the freezer while you make the chocolate topping.
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18
In a small saucepan, heat the 1 cup of cream and once it comes to a boil pour it over the chopped semisweet chocolate and whisk to make a smooth ganache.
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19
Do the same with the remaining 1/2 cup of cream and the white chocolate, adding the mint extract and green coloring to make a light mint green color then place it in small pastry bag with a small plain tip.
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20
Remove the cake from the freezer and working quickly spread the chocolate ganache over the surface of the cake smoothing it with an offset spatula.
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21
Drizzle lines of the green ganache over the entire surface of the cake at diagonal.
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22
Take a toothpick and run it back and forth in the opposite direction to draw it through the ganache and make decorative pattern.
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23
Return to the freezer until ready to serve.
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24
Trim off the edges to even off the cake then cut into rectangle pieces.