4-8 slices salted butter, to sandwich between the bread
Directions
Dissolve the instant coffee in the hot water and let it cool slightly.
Make the spread: cream the butter and sugar, beat in the egg, then the dissolved coffee and vanilla. Mix in the flour and a pinch of salt to a smooth, pipeable paste.
For each toast, lay slices of salted butter in the middle of a slice of bread and press a second slice on top to sandwich the butter in the centre.
Brush the tops with eggwash, then pipe or spread the coffee butter all over the top of each sandwich.
Bake at 180C (355F) for 12-15 minutes, until bronzed and fragrant with a melty, buttery centre.