Preheat your oven to 350˚F. Coat your muffin tins with a non-stick spray or if you prefer, place a muffin liner in each.
Prepare the carrot cake batter as directed in my carrot cake recipe (see link above). Fill the muffin tins 3/4 full with the batter.
Prepare the streusel topping: combine the flour, sugar, cinnamon, nutmeg and salt together in a large bowl. Add the small cubes of butter and using your hands, rub the ingredients together until you have a coarse, crumbly mixture. Scatter this mixture over top of each batter-filled muffin tin until you have used it all. Bake in the oven for 15-20 minutes or until a toothpick inserted comes out clean. Once cooked through, remove from the oven and leave to cool for 5 minutes. Gently remove each muffin from the tin and leave to cool on a wire rack.