Preheat oven to 350°F (175°C). Grease a 9x13-inch baking pan and line with parchment paper, leaving overhang.
Melt butter in a saucepan over medium heat. Add brown sugar and corn syrup, stirring until sugar dissolves.
Remove from heat and stir in coconut and pecans until well combined.
Spread mixture evenly in the prepared pan.
Melt chocolate with water in a microwave-safe bowl in 30-second intervals, stirring between each, until smooth. Let cool slightly.
Whisk together flour, baking soda, and salt in a medium bowl.
Beat butter and sugar with an electric mixer until light and fluffy, about 3-4 minutes.
Add eggs one at a time, beating well after each addition. Stir in vanilla and melted chocolate mixture.
Gradually add flour mixture alternating with buttermilk, beginning and ending with flour mixture. Mix until just combined.
Pour batter over the prepared topping, spreading evenly.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Let cake cool in pan for exactly 10 minutes.
Run a knife around the edges to loosen the cake.
Place a serving platter over the cake pan and carefully invert.
Gently peel off parchment paper if needed. Let cool completely before serving.