- Make the pudding: Preheat convection oven to 325 degrees (or conventional oven to 350 degrees). Butter a well-seasoned 12-inch cast-iron skillet. In small sauté pan, melt 1⁄4 cup butter over medium-high heat and toast chestnuts until slightly brown, approximately 8 minutes, tossing frequently. Add cinnamon and nutmeg, and cook 1 minute longer until spices are toasted and aromatic. Set aside to cool.
- In a saucepot, bring dates and 4 cups cold water to a boil. Drain dates and transfer to a large bowl, stir in baking soda, cover, and let rest for 15 minutes, until they’ve softened and started to break down. (Green foam will appear in date mixture—don’t be alarmed.) Blend with a standard blender or immersion blender until smooth. Cool to room temperature.
- In the bowl of a stand mixture fitted with paddle attachment, beat remaining 1 cup butter, sugar, and salt on high speed until creamy. Slowly add one egg at a time, scraping sides of bowl as necessary. This mixture will look granular and broken. Slowly add cooled date puree, orange zest, and chestnuts to the stand mixer, and mix on low speed until incorporated.
- Sift together flour and baking powder and fold into date-chestnut mixture with rubber spatula until incorporated. Pour batter into prepared cast-iron skillet and bake for 45 minutes (or 1 hour if using a conventional oven), or until top is golden brown and cake tester inserted into the middle of pudding comes out clean and center is firm.
- Make the sauce: In a large saucepan, combine all ingredients over medium heat and whisk constantly until thick, approximately 10 minutes.
- To assemble: When pudding is fully cooked, pour warmed sauce over the top and allow to rest and absorb for 5 minutes. Garnish and serve immediately.