1) Preheat the oven to 350°F. Lightly grease a 9" x 13" pan; or line with parchment, and grease the parchment.
2) To make the cake: Add the boiling water to the cocoa a bit at time, stirring until well blended. Set aside to cool to room temperature.
3) Whisk the remaining dry ingredients together in a mixing bowl. Add the eggs and oil; beat on medium speed for 2 minutes.
4) Scrape the bottom and sides of the bowl, and add the vanilla. Then the cocoa mixture in three additions, scraping the bottom and sides of the bowl after each addition. Pour the batter into the prepared pan.
5) Bake the cake for 30 to 35 minutes, until a cake tester inserted into the middle comes out clean. Remove from the oven and set on a rack to cool, while you make the topping.
6) To make the topping: Whisk together the brown sugar, ClearJel or cornstarch, and salt until thoroughly combined.
7) Melt the butter in a medium-sized saucepan; stir in the milk or half and half, sugar mixture, and corn syrup.
8) Bring to a simmer and cook gently, stirring constantly, until the sugar is dissolved and the mixture thickens slightly, about 1 to 2 minutes.
9) Remove from the heat and stir in the vanilla, flavor, coconut, and pecans.
10) Spread the topping onto the warm cake. Let rest at room temperature for about 1 hour, until the topping sets. Serve in squares.
Yield: 24 servings.