Grease and line a 22cm springform cake tin. Set your slow cooker to low.
Combine the butter and chocolate in a heatproof bowl and place over a saucepan of gently simmering water. Stir until melted and glossy, then remove from the heat and mix through the vanilla, vodka and espresso. Stir to combine, then set aside.
Add the sugars and eggs to the bowl of a freestanding electric mixer fitted with the whisk attachment and beat on high until pale,thick and creamy, about 4 minutes. Stir through the buckwheat flour and cocoa powder. Slowly pour in the chocolate mixture and stir to combine, being careful not to overwork the batter.
Gently pour into the prepared tin. Cut yourself a length of baking paper about 30 × 15 cm to act as handles underneath the cake tin. This will help you lower the tin into the cooker and get it out again. Transfer the tin to the slow cooker then cover with a tea towel. This will prevent any moisture from the cooker dripping onto the torta. If your slow cooker has a removable lid, set it slightly ajar. If it is attached and twists closed, merely set it down but do not lock it into position as you want some of the steam to escape.
Cook for 2 ½ hours, then check its progress. Touch the top: if it feels lightly set and tacky to the touch but firmer around the edges, remove the tea towel and the lid and cook for an additional hour (if not, check on it again at 10-minute intervals). Turn the cooker off and, gently, using your handy handles, remove the tin from the bowl. This torta is quite delicate, so pop it in the fridge for 30 minutes to set before slicing.
When ready to serve, dust with cocoa powder and slice. Allow to come to room temperature before serving. Top with crème fraîche and dried orange slices, if using.