Pour 480 ml of apple cider into a small saucepan and 240 ml into a second saucepan. Simmer both over medium heat until reduced to 120 ml and 80 ml respectively. Remove from heat and allow both cider reductions to cool to room temperature.

Preheat your oven to 177 °C (175 C). Generously grease a 10 to 2880 ml bundt pan with softened butter or baking spray with flour, ensuring all fluted crevices are fully coated.

In a large bowl, whisk together all-purpose flour, baking powder, baking soda, fine sea salt, 1.5 teaspoons of ground cinnamon, nutmeg, and allspice until evenly combined.

In a stand mixer fitted with the paddle attachment, beat 240 ml softened butter, 240 ml granulated sugar, and packed brown sugar on medium-high speed for 3 to 4 minutes until pale and fluffy. Add eggs one at a time, beating well after each, then blend in vanilla extract.

In a separate measuring jug, whisk the cooled 120 ml cider reduction, buttermilk, and unsweetened applesauce. With the mixer on low speed, add the dry ingredients in three batches, alternating with the liquid cider mixture in two batches, mixing just until streaks disappear.

Spoon the batter into the prepared bundt pan and smooth the top with an offset spatula. Bake for 42 to 48 minutes, or until a wooden skewer inserted into the center comes out clean with just a few moist crumbs attached.

While the cake bakes, make the caramel. In a heavy-bottomed saucepan, melt 180 ml granulated sugar over medium heat without stirring until amber, then carefully whisk in 60 ml cubed butter and the 80 ml reduced apple cider. Slowly pour in warmed heavy cream, simmer for 1 minute, and stir in flaky sea salt. Set aside to thicken.

Cool the cake in the pan for 15 minutes, then invert onto a wire rack. While still warm, brush the entire surface with 60 ml melted butter. Stir 158 ml granulated sugar and the remaining 0.5 teaspoon cinnamon in a shallow dish and press or spoon the cinnamon sugar all over the cake.

Transfer the coated cake to a serving platter. Drizzle generously with warm apple cider caramel sauce, saving the remaining caramel in a pitcher to serve alongside each slice.