Make the Streusel

1. In a bowl, combine flour, brown sugar, cinnamon, and salt. Add cooled melted butter and toss with a fork until the mixture becomes crumbly and sandy.
2. Chill until needed.

Make the Muffins

1. Preheat the oven to 220 ℃ fan (425℉) and line a muffin tin with 6 tulip muffin cases.
2. Melt the butter in a saucepan over medium low heat.  Set aside and allow it to cool completely.
3. In a small bowl, dissolve the instant coffee powder in hot water. Stir until fully dissolved and smooth. Leave to cool to room temperature before adding to the batter.
4. Place the chopped walnuts in a small bowl and lightly mist them with water (a spray bottle works best). Add about 2 tablespoons of flour and toss well to coat each piece.
5. In a mixing bowl, whisk melted butter, brown sugar, and caster sugar until well combined. Add the egg, cooled coffee mixture, sour cream, and vanilla extract; whisk until smooth.
6. Sift in the remaining flour, baking powder, and salt. Whisk until just combined. Fold in flour-coated walnuts with a spatula.
7. Fill each liner approximately to the top of the cake pan level (about 1½  medium ice cream scoops of batter). Spoon chilled streusel generously over the top.

Bake and Cool the Muffins

1. Bake in a preheated oven at 220°C fan for 5 minutes, then reduce the heat to 180°C fan and bake for another 13–15 minutes, or until a skewer inserted comes out clean.
2. Let the muffins cool slightly in the pan, then transfer to a wire rack to avoid soggy bottoms.
3. Peel back the tulip liners and enjoy while still warm with a cup of coffee.