Preheat the oven to 170ºC/ 150ºC (fan)/ gas 3. Brush a 12-hole cupcake tin with butter.
First, warm the walnuts through on a baking tray in the oven. Do not toast them, you just want to bring out the fragrant oils. This should take less than 5 minutes. Let the nuts cool slightly then chop fine. Set aside.
In a large bowl, sift together the flour, baking powder, bicarbonate of soda, salt and spices, then whisk this mixture through the chopped nuts. Set aside.
In an electric mixer, whisk the butter and caster sugar until light and fluffy.
Add the eggs one at a time, mixing until each one is fully incorporated, then add the vanilla extract. Mix in the flour and nut mixture and then the crème fraîche.
Divide the batter between the 12 tins and bake for 20 minutes until the cakes spring back to the touch. Let the cakes cool in their tins for about 10 minutes, then gently pop them out (you may need to run a small paring knife around the inside of the tins to ease the cakes out). Place the cooled cakes upside down on a wire rack.
Whisk together the ingredients for the icing and spoon it over the cakes. Use the back of a spoon to gently guide it to the edges so that it willingly drips down the sides.