Place oven rack in centre of oven; preheat oven to 325°F. Line an 18- x 13-inch baking sheet with parchment paper without greasing pan.

In bowl, whisk together all-purpose flour, rye flour, baking soda and smoked salt. In large bowl, whisk together olive oil, sugar and cocoa powder; whisk in hot coffee until incorporated. Whisk in eggs, one at a time, until combined.

Add flour mixture and whisk until smooth, scraping bottom and sides of bowl. Add sour cream and vanilla, and whisk until batter is smooth. Scrape into prepared pan, keeping parchment in place; firmly tap pan on counter a few times to remove any air bubbles.

Bake until a toothpick inserted in centre comes out clean and cake springs back when touched, about 15 to 16 minutes. Let cake cool for 15 minutes; run a paring knife around cake edges to release it from pan. Place a large piece of parchment paper over cake; cover with another baking sheet or tray, then invert. Remove top pan and peel off parchment paper to reveal cake. Let cool.

Chop chestnuts into small pieces; place in a saucepan with milk, sugar, vanilla and salt. Bring to a low boil over medium heat, stirring frequently; reduce heat and simmer, stirring occasionally, until chestnuts are softened and liquid is reduced by a quarter, about 15 to 20 minutes.

Transfer chestnut mixture into blender and purée, staring on low and then increasing to high, until silky smooth. Add chocolate and blend again, scraping sides, until incorporated. Scrape into large shallow bowl, whisking a few times until cooled to room temperature.

Using electric mixer, beat whipping cream in large bowl until soft peaks form. Fold one-third of the whipped cream into chestnut mixture. Fold in the remaining whipped cream, being careful not to overmix.

Assembly: Using large offset spatula, evenly spread chestnut mousse to edges of cake while it’s still on baking sheet. This will be important when stacking the layers later. Refrigerate until set, at least 3 hours. Loosely cover with plastic wrap.

Slide parchment and cake from baking sheet onto large cutting board or work surface. Using chef’s knife, trim about ½ inch from each side to achieve clean lines. (Tip: Use kitchen twine or dental floss to gently mark a straight line on the top of the mousse before cutting.)

Cut cake in half lengthwise and then crosswise, wiping knife clean between each cut, to create four equal rectangles. Using a large cake lifter or barbecue spatula, lift first cake rectangle, ensuring cake is released from parchment, and transfer to platter. Repeat and stake with remaining 3 cake layers, being careful to position accurately before stacking. Refrigerate.

In small saucepan, heat cream until almost simmering. Remove from heat; add dark chocolate and let stand for 1 minute. Whisk until smooth; let cool completely.