Preheat oven to 180 degrees C. Grease and flour a 23 x 33cm cake tin.
Streusel Topping: In a small bowl combine the flour and brown sugar. Rub in the peanut butter and butter between fingers until mixture resembles coarse crumbs. Set aside.
Cake: In a large bowl mix together flour, brown sugar, baking powder, bicarbonate of soda and salt. Make a well in the centre and pour in the milk, eggs, peanut butter and butter. Mix well and pour into prepared tin. Spread evenly then sprinkle streusel over top.
Bake in the preheated oven for 30 minutes or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.