Brew half a cup of strong coffee and divide into 2 cups. Dissolve 30 g of sugar (2 tbsp) in one cup and leave the other cup unsweetened. Let cool.
Preheat the oven to 180º C.
Grease or line a 20 cm cake tin with parchment paper.
Crack and chop walnuts.
Beat butter and 120 g of sugar until pale. Add one egg at a time while still beating.
Sprinkle mixture with walnuts (or beat 3 tbsp hazelnut butter into the mixture).
Sift flower and baking powder over the egg mixture and pour the unsweetened 1/4 cup coffee on top. Quickly fold everything together.
Bake for 25 minutes or until a skewer inserted into the middle comes out clean.
Meanwhile, beat whipping cream with maple syrup. Mix sour cream with the sweetened 1/4 cup coffee and combine both mixtures. Chill coffee cream in fridge while the baked cake is completely cooled.
Cut the cake lengthwise and spread half of the coffee cream onto the bottom half. Put other half back on and top with remaining coffee cream.