Preheat the oven to 180C. Grease and line the base and sides of 2 20cm cake tins.
Make the chocolate ganache: Combine the cream and Baileys in a saucepan and bring to a simmer over medium heat. Place the chocolate in a heat resistant bowl, pour over the hot cream mixture and stir until the chocolate is fully melted and the mixture smooth. Set aside to cool to room temperature. The ganache will thicken as it cools.
While the ganache cools, prepare the cake.
Make the cake: Sift the flour, cocoa, bicarb, baking powder and salt into a large bowl. In a separate bowl, combine the eggs and sugar, and beat for 2-3 minutes until pale and thick. Add the oil and sour cream, and beat well to combine. Fold in half of the dry mix, followed by the coffee, then the remaining dry mix, being careful not to overmix between each addition. Finally, add the Baileys Salted Caramel and vanilla, and stir briefly to ensure the mixture is smooth and well combined. The mixture will be quite liquid at this point.
Divide the mixture between the prepared cake tins and bake in the middle of the oven for 45 minutes, or until a skewer inserted into the middle of the cake comes out clean.
Cool in the tins for 10 minutes, then turn out onto a wire cooling rack and set aside to cool completely.
Make the caramel: In a saucepan, combine the sugar and water, place over a low heat, stirring occasionally, until the sugar dissolves. Once no sugar granules remain, increase the heat to high, and bring to a boil. Continue to boil until the caramel turns a deep amber. Remove from heat, and pour in the cream and Baileys a little at a time, mixing well between additions to achieve a smooth rich caramel. Add the butter and stir to combine.
Set aside to cool.
Assemble the cake: Using a serrated knife, trim the tops of the cakes to create a flat surface. Crumble the off-cuts into a bowl and set aside for decorating. Halve each cake horizontally to create 4 even discs.
Place a layer of cake on a cake stand or serving plate, securing it with a little ganache. Spread a thin layer of caramel over the surface, followed by some of the ganache. Repeat with the remaining layers, keeping the layers even and straight as you go. Spread the remaining ganache over the top and sides of the cake, using a cake scraper to create a smooth surface. Use the reserved cake crumbs to decorate.