Toast all the walnuts (if not already toasted), including the halves reserved for decoration by spreading on a baking sheet and placing in a pre-heated oven at 190 C/375 F/Gas 5 for about 10 minutes, turning a couple of times during the process to make sure they are evenly toasted. Leave to cool.
Once cool, remove 12 good-shaped half walnuts and set aside for later decoration.
Grind the remaining walnuts in a grinder/food processor so that they have the texture of coarse sand (watch closely to make sure they do not grind too far and end up as a paste)
Weigh and whisk together the ground walnuts (75g ground weight), flour, xanthan gum, baking powder, bicarbonate of soda, and salt, making sure all lumps are broken down. TIP : weigh into an airtight container and shake vigorously to blend. Set aside.
Base line two 20 cm/8 inch baking tins with baking paper and pre-heat the oven to 180 C/350 F/Gas 4.
Cream together the butter and brown sugar with an electric whisk until pale and fluffy.
Break the eggs into a small bowl and lightly beat with a fork to combine. Gradually add to the butter mixture a little at a time, beating thoroughly between each addition.
Add the espresso powder and yoghurt and beat again until even in colour.
Carefully fold in the dry ingredients until evenly combined (but be careful not to over-mix).
Split the cake batter equally between the two tins and spread to the edges, making sure the top is smooth and level.
Bake for 25 to 30 minutes until golden, the top springs back to the touch and a skewer inserted comes out clean.
Remove from the oven and allow to cool in the tins for 10 minutes, before turning out onto a wire rack to cool completely.
Slowly mix together the espresso powder and boiling water in a small bowl until smooth. Set aside.
Beat the butter until soft and pale.
Gradually add the icing sugar (a little at a time), espresso mixture and the salt. The frosting should be well blended and a smooth spreadable consistency. Add a little milk if the consistency needs to be loosened.
Use some of the frosting to sandwich the sponges together and then spread the rest on the top and sides of the sandwiched cake as you wish. Smooth with a cake spatula.
Decorate with the reserved toasted walnut halves, the chocolate-espresso beans and a light sprinkling of instant espresso powder.
Store in an airtight cake tin/carrier at room temperature.
* Note: nutritional information is an estimate & may vary according to portion size/ingredient variants.Gluten Free Alchemist Rice Free Flour Blend A can be found on the GFA Flours & Flour BlendingPage.