Preheat oven to 180°C. Line a muffin tin with paper. To make the cupcakes, beat the butter, sugars and eggs together. Mix in other ingredients. Divide mixture into the cupcake liners. Bake for 18-20 minutes. Cool in tin until cold.
Ginger Crumble Topping
Preheat oven to Using hands, mix ingredients. Spread the mixture onto a baking sheet lined with parchment paper. Bake at 175°C for 20 minutes. Crumble any pieces that are stuck together.
For the tall muffin case cut a piece of baking paper 20cm x 12cm, spray edges with baking spray and overlap to form a seal. Place inside a Texas muffin paper cup then fit both into a ramekin about 6 ½ cm in diameter or a ring mould of this diameter will also work. Take table spoons of muffin mixture and drop from above the top of the paper carefully so as not to get any on the top section of paper. Fill to just below level of top of ramekin sprinkle with ginger crumble topping (approximately 2 teaspoons per cup cake) and bake as normal. Bake around 30 mins.
Place the butter, icing sugar, lemon zest and juice in a mixing bowl and beat together until well blended, smooth and fluffy.
Peanut Butter Cream Cheese Frosting
Beat all ingredients (except walnuts) until smoothly combined using mixer. Spread on cold cupcakes and sprinkle with chopped walnuts.