Heat the oven to 170C/335F/gas mark 3. Spread the walnuts out on a baking tray, and cook for 10 minutes. When they're cool enough to touch, chop them into 1/2cm sized pieces.
Next make the brown butter. Place the butter in a small saucepan and cook over a medium heat until melted. Continue to cook until the butter is foaming, swirling the pan so the solids brown more evenly. Leave the butter to bubble away until it turns a rich golden brown, then take off the heat and leave to stand for five minutes. Strain through a sieve, discarding the solids, then leave to cool slightly.
The brown butter should still be warm when you fold it into the mix, too hot, it will “cook” the egg whites; too cool, it will be hard to incorporate.
While the butter is cooling, sift the icing sugar, flour, baking powder and salt into a large bowl, then whisk in the ground almonds.
Take the coffee granules and dissolve them in the boiling water and place to one side.
Place the egg whites in a bowl and use a whisk to froth them up a little, pour the egg whites and dissolved coffee granules into the dry ingredients, stir until just combined.
Add the browned butter and mix until the batter is thick, shiny and smooth. Fold in the walnuts and ground coffee, then cover with cling-film and refrigerate for at least two hours.
Heat the oven to 200C/390F/gas mark 6. Butter the moulds of your chosen muffin tin, dust with flour and tap away any excess. Spoon the batter into each mould, depending on the size of your muffin tin they will be almost full - but they don't rise too much. Bake them for about 25 minutes, test with a skewer and if it comes out clean they're cooked.
Make the icing while the financiers are baking. Sift the icing sugar into a medium bowl, dissolve the coffee in the hot milk and add to the icing sugar and mix until smooth, then set aside.
Remove the tin from the oven, set aside the financiers to cool for five minutes then turn the cakes out of the tin and put the financiers on a rack to cool.
To serve, spread the icing on top and finish each financier with a walnut half. These keep well for about 3-4 days in an airtight container at room temperature.