Prepare the Cake: Preheat oven to 350°F. Coat 2 (8-inch) round cake pans with baking spray.

Place boiling water and mint sprigs in a heatproof bowl. Let steep 10 minutes. Discard mint, and stir in chocolate chips; let stand 2 minutes. Stir until chocolate is melted, about 1 minute. Whisk in oil, sour cream, eggs, and vanilla until smooth.

Stir together flour, sugar, cocoa, baking soda, salt, and baking powder in a large bowl. Stir in wet ingredients. Pour batter evenly (about 2 cups per pan) into prepared pans.

Bake in preheated oven until a wooden pick inserted in center comes out clean, 20 to 25 minutes. Cool in pans 10 minutes. Remove cake layers to wire racks, and let cool completely, about 1 hour.

Prepare the Frosting: Place butter and mint leaves in a small saucepan over low. Cook, stirring often, until butter is melted and begins to foam, about 15 minutes. Remove from heat; discard mint. Transfer mint butter to a heatproof bowl, and refrigerate until re-solidified but still soft, about 1 1/2 hours.

Beat mint butter in bowl of a stand mixer fitted with a paddle attachment on medium-high speed until light and fluffy, about 2 minutes. Reduce speed to low, and add powdered sugar, vanilla, salt, and 2 tablespoons of the cream until combined. Increase speed to medium-high, and beat until fluffy, about 2 minutes, adding remaining cream, 1 teaspoon at a time, to reach desired consistency if needed. Beat in 1 drop liquid food coloring, adding more, if necessary, until desired color of pale green is reached.

Spread 1 cup frosting between cake layers; spread a thin layer on top and sides of cake, smoothing with a spatula. Chill cake and remaining frosting 30 minutes. Spread remaining chilled frosting on top and sides of cake. Chill cake until firm enough to cut, about 1 hour. Garnish with mint sprigs, if desired.