1. Preheat the oven to 375 F (190 C). Grease a 10-cup (2.5 L) bundt pan and dust it with flour, tapping out any excess.
2. Using electric beaters or a stand mixer fitted with the paddle attachment, beat the butter, granulated and brown sugars until smooth. Add the eggs one at a time, beating well after each addition (and scraping the bowl, if needed).
3. In a separate bowl, sift the flour, baking powder, cinnamon, ginger, nutmeg, baking soda and salt. Add half of this to the butter mixture and beat until combined. Add the pumpkin puree and beat until smooth, finishing with the second half of the flour, beating until smooth. Stir in the chocolate chips by hand. Spoon the batter into the prepared pan and spread to level it. Bake the cake for 5 minutes at 375 F (190 C) and then reduce the oven temperature to 325 F (160 C) and cook for an additional 60-75 minutes, until; a tester inserted in the cake comes out clean. Cool the cake in the pan for 20 minutes on a rack, then tap out to cool completely.
4. Repeat this recipe to bake a second bundt cake. Once both cakes are cooled, they are ready to be frosted.
5. For the buttercream, whip the egg whites using electric beaters or in the bowl of a stand mixer fitted with the whip attachment on high speed until they are frothy and then slowly pour in ½ cup (100 g) of sugar and continue to whip until the whites hold a soft peak when the beaters are lifted.
6. Bring the remaining 1½ cups (300 g) of sugar and the water up to a full boil over high heat without stirring. Cook the sugar until it reaches 240 °F (116 °C) on a candy thermometer. With the beaters going, carefully and slowly pour the sugar syrup down the side of the bowl (to prevent splashing) and whip on high speed until the meringue has cooled, about 8 minutes.
7. With the mixer still running on high, add the butter a few pieces at a time and continue to whip until the buttercream is smooth. (It may look craggy or even curdled as it blends, but just keep mixing – it’ll all smooth out). Mix in the vanilla. Add the food colouring paste a little at a time and stir in by hand until you reach the desired colour tone.
8. To assemble the pumpkin cake, spoon a few cups of the buttercream into a piping bag fitted with a large plain tip. Trim the flat side of each bundt cake (to make them level) and reserve this trim. Place the first cake, curved side (normally the top of a bundt cake) on the bottom, on a cake board. Use a 2 ¾ -inch (7 cm) and 2-inch (5 cm) round cookie cutter and cut rounds from the reserved cake trim. Place the large piece in the centre at the bottom of the cake (lift the bundt up, to fit it under) and pipe a layer of frosting over it. Drop the smaller circle of cake overtop and pipe frosting on this – it should now be level with the top of the cake (or cut a third circle of cake, if needed). Spread a layer of buttercream over the cake and place the second bundt on top, so that the flat sides meet – you should see the pumpkin take shape now. Repeat as before, filling the hole of the bundt with cake cut-outs and frosting.
9. To cover the cake, pipe a ring of frosting around the base of the pumpkin, then pipe frosting upwards, leaving space between the pipings. (This technique makes it easier to spread the frosting over the curves of the cake evenly.) Spread the frosting to cover the cake completely.
10. For the stem, stir 1-2 Tbsp (8–15 g) of cocoa powder into 1 cup (250 mL) of the remaining buttercream. Cut two more 2-inch (5 cm) round pieces of cake from the trim and stack these on top of each other, adhering with frosting. Cover this with the chocolate buttercream and place this on top of the pumpkin. Use a fork to create a striated pattern up the stem. Roll on the green fondant (using icing sugar, not flour, when rolling) and cut a few leaves, to place at the base of the stem. Shape some of the fondant into a thin rope and wrap it around a pencil or dowel and let this dry, to make tendrils to place on. Chill the cake until ready to serve, but the cake can sit out for up to 6 hours on display.