Preheat oven to 350°F (175°C). Grease and line an 8x8-inch pan with parchment.
In a small bowl, mix flour, brown sugar, cinnamon, and salt into your melted butter. Chill until ready to use. (It needs time to firm up so it's able to crumble properly; so make sure you make your streusel before starting anything else.)
In a medium bowl, whisk flour, baking powder, baking soda, salt, spices, pecans, and coconut.
In another bowl, whisk melted butter and both sugars. Then whisk in your two eggs and vanilla. Then lastly, whisk in buttermilk until smooth. Stir in grated carrots. Add dry ingredients and fold until just combined.
Pour batter into prepared pan and smooth the top. Sprinkle chilled streusel evenly over the surface.
Since our cake is a bit more delicate and airy than a tradition coffee cake, you'll notice the streusel sink in a bit once baked; that's completely fine!
Bake for 40 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Let cool in the pan for 15–20 minutes.
Whisk softened cream cheese with powdered sugar, buttermilk, and vanilla until smooth and pourable. Drizzle over slightly warm or cooled cake.