Preheat oven to 200 degrees C. Grease a bottom a 30cm x 40cm large slice tray or baking dish. line with baking paper.
Sift white sugar and flour together in a bowl.
Combine hot water, smooth peanut butter and butter in a saucepan; bring to a boil, stirring frequently. Remove from heat.
Blend yogurt, eggs and bicarb together in a bowl. Pour peanut butter mixture into egg mixture in a steady stream, stirring constantly. Mix flour mixture into peanut butter mixture until batter is combined. Spread batter into prepared pan.
Bake in preheated oven until the center of the cake springs back when touched lightly with a fingertip, about 20 minutes. Cool on wire rack.
Meanwhile, make the Frosting:
Combine crunchy peanut butter, butter and milk in a small saucepan; bring to a boil, stirring constantly. Reduce heat to low; stir in icing sugar and maple syrup until sugar dissolves and frosting is smooth. Pour warm frosting over warm cake. Sprinkle walnuts, pecans and almonds over the top; cool completely.
I use creamy peanut butter in the cake, and chunky peanut butter in the frosting, but you can use whichever type you prefer.