Make the pumpkin purée. Preheat the oven to 400°F and line a baking sheet with parchment paper. Cut the pumpkin in half from top to bottom. Scoop out the seeds using a spoon or ice cream scoop. Sprinkle the flesh of the pumpkin lightly with salt and set flesh side down onto the prepared baking sheet. Bake for 45 minutes to an hour (depending on how large your pumpkin is). It will be done when the skin and flesh is easily pierced with a knife. Remove from the oven and cool completely. Scoop out the pumpkin flesh and put it in a blender or food processor. Blitz it until it’s completely smooth. Store, sealed in a container, in the refrigerator for up to one week.
Make the prune purée. Combine 8 ounces of pitted prunes with 1/4 cup of hot water. Let sit for 5-10 minutes to soften prunes slightly. Combine the prunes and water in a blender. Pulse to combine, then blend until smooth, pourable consistency forms, scraping the sides if necessary. Store the purée in an airtight container in the fridge for up to 4 weeks.
Preheat oven to 325°F and grease a 9″ baking pan with butter or gluten-free cooking spray. Line the pan with parchment, allowing the paper to drape up two opposite sides of the pan. Spray parchment with cooking spray. Set aside.
In a large bowl, cream butter and brown sugar until light and fluffy, about 5 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in pumpkin, molasses, prune puree and orange zest.
In a small bowl, combine flour, baking soda, ginger, cinnamon, cloves, salt and ground pepper. Add to creamed mixture alternately with buttermilk, beating well after each addition.
Pour batter into a greased 9-in. square baking pan. Bake until a toothpick inserted in center comes out clean, 28-32 minutes. Cool completely on a wire rack.
Make the glaze. Heat the butter and brown sugar together in a medium saucepan. Boil for 1 minute, whisking constantly, and remove from heat. Stir in the vermouth, vanilla and powdered sugar and beat with a whisk until smooth. Drizzle the glaze evenly over the cake. Store the cake at room temperature for up to three days.