Preheat oven to 350F and line an 8x8 inch baking dish with parchment paper.
Mix all the ingredients for the crumble together until it has a wet sand texture and holds together. Set aside
Whisk together the dry ingredients for the carrot cake in a medium bowl.
In the bowl of a stand mixer or in a large bowl with an electric mixer, beat together the wet ingredients, except the carrots.
Add the dry ingredients to the wet and mix until just combined and a few streaks of flour remain. Fold in the carrots and any add ins.
If adding the cinnamon-sugar swirl, pour half the batter into the prepared baking dish then sprinkle the cinnamon sugar over top. Evenly spread the remaining batter over top then use a knife to gently swirl in the cinnamon-sugar. Alternatively, spread all of the batter evenly into the prepared baking dish.
Evenly crumble the streusel topping over the cake.
Bake for 40-45 minutes until a toothpick inserted in the center of the cake comes out with a few crumbs and the top is golden brown.
Cool on a wire rack before icing.
If icing, mix together the powdered sugar with milk until you have a smooth icing. Add more milk as needed.