Heat oven to 350°F. Grease and line two 9-inch round cake pans with parchment.
Whisk flour, cocoa, sugar, baking soda, and salt in a large bowl.
Stir buttermilk, oil, and vanilla in a measuring jug. Pour in hot coffee and mix.
Combine wet with dry. Whisk until smooth and no dry streaks remain.
Divide batter between pans. Bake 28–33 minutes, until a toothpick comes out clean.
Cool cakes 10 minutes in pans. Turn out to racks and cool completely.
Make a slurry by whisking cornstarch with 2 tbsp evaporated milk in a small bowl.
Warm remaining evaporated milk, brown sugar, butter, and salt in a saucepan over medium heat.
Whisk in the slurry. Cook, stirring, until thickened and bubbling, 4–6 minutes. Remove from heat.
Stir in coconut, pecans, and vanilla. Cool until spreadable, about 15 minutes.
Assemble: Place one cake layer on a plate. Spread half the frosting on top. Add second layer and finish with remaining frosting.