How to Make It
Preheat oven to 350°F. Fill a medium pot with water; bring to a boil. Line a 13-by-9-inch pan with a length of foil, letting foil edges hang over at least 2 inches on longer sides of pan. Grease and flour pan. In a large bowl, whisk together flour, baking soda and salt. In a separate bowl, stir together cocoa powder and 1/3 cup boiling water; whisk in milk until smooth.
Using an electric mixer on medium-high speed, beat butter and sugar until fluffy. Beat in eggs, 1 at a time, and vanilla. Reduce speed to low; add flour and cocoa mixtures, alternating each 3 times, scraping down sides of bowl.
Pour batter into prepared pan and, using a rubber spatula, spread smoothly. Bake, rotating halfway through, until a toothpick inserted into center comes out clean, about 35 minutes. Cool in pan for 1 hour; using foil as handles, remove cake from pan and turn out onto a tray to cool completely.
Color leaves: In a small bowl, combine sliced almonds and pumpkin seeds with 3 drops of red coloring. Gently mix with a spoon. Repeat with drops of yellow and brown coloring, as desired.
Decorate cake: Place 2 Tbsp. frosting in a ziplock bag. Cut a small hole in corner and pipe RIP onto cookies. Cover top and sides of cake with remaining frosting. Adhere pretzel sticks along sides, about 1/4 inch apart, leaving 2 2-inch gaps for path. Create path with candy corn.
Cut 7 small slits on top of cake and insert gravestone cookies; scatter almonds and pumpkin seeds at each cookie, and at corners and edges of cake, to resemble fall leaves. Add candy pumpkins around cake, as desired.