For the devil's food cake: Preheat the oven to 325 degrees F. Grease a 9-by-13-inch cake pan and line the bottom with parchment with an overhang on the longer side. Grease the parchment.
Combine the chocolate, cocoa and instant coffee in a large bowl. Pour the hot water over to melt; let sit 2 to 3 minutes and then whisk until smooth. Let cool slightly.
Add the sugar, flour, baking soda, baking powder and salt to a stand mixer fitted with the paddle attachment and mix on low to combine.
Add the buttermilk to the chocolate mixture to cool it slightly more. Add the eggs, oil, vanilla and chocolate-buttermilk mixture to the stand mixer. Turn the mixer on low and combine until the dry ingredients are moistened. Increase the mixer speed to medium and beat for 2 minutes, scraping halfway through.
Pour the batter into the prepared pan and bake until a toothpick comes out with a few moist crumbs, 58 to 60 minutes. Allow the cake to cool completely in the pan, then wrap the whole thing in plastic wrap and chill for 2 hours in the refrigerator.
Unmold the by unwrapping the plastic, placing a fresh piece of parchment and a wire rack on top of the cake, then turning the cake out of the pan. (Cold sheet cakes are easier to unmold and have less chance of breakage.) Flip the cake onto a serving board. Trim the sides and level if necessary.
Mix the buttercream with 2 parts green and 1 part yellow gel food coloring until it's tinted light green. Transfer about 1 1/2 cups of the buttercream to a small microwave-safe bowl and microwave it about 5 seconds to soften. Frost the top of the cake and then the sides with the warmed buttercream. Use a scraper to square off the edges. Chill in the refrigerator until the buttercream is no longer sticky, 20 to 30 minutes. Frost the cake a final time and chill in the refrigerator until the buttercream is firm, about 30 minutes.
Fit a pastry bag with a grass tip. Divide the remaining green buttercream between two small bowls and darken 1 bowl with more green and yellow food coloring. Fill the pastry bag with the two colors, allowing them to mix slightly, then pipe grass on the top of the cake.
Cut the rolled wafer cookies pieces in half so they are about 1/2-inch higher than the top of the cake. Attach to the sides of the cake with a little bit of extra buttercream, leaving some space in between. Place the chocolate eggs in the grass.