Heat the oven to 350°F. Lightly grease a 9-by-13-inch pan, or line it with parchment and grease the parchment. Stir the boiling water into the cocoa a little at a time until blended. Set aside to cool to room temperature.

Whisk the granulated sugar, flour, 2 tablespoons cornstarch, baking powder and 1 teaspoon salt together. Add the eggs and oil and beat on medium speed for 2 minutes. Scrape the bowl and add 2 teaspoons vanilla. Add the cocoa mixture in three additions, scraping the bowl after each.

Pour the batter into the pan. Bake for 30 to 35 minutes, until a cake tester inserted into the middle comes out clean. Set the pan on a rack while you make the topping.

Whisk the brown sugar, remaining cornstarch and remaining salt together. Melt the butter in a medium saucepan, then stir in the half-and-half, sugar mixture and corn syrup. Bring to a simmer and cook gently, stirring constantly, for about 1 to 2 minutes, until the sugar dissolves and the mixture thickens slightly.

Remove the saucepan from the heat. Stir in the remaining vanilla, toasted coconut and toasted pecans. Spread the topping over the warm cake.

Let the cake rest at room temperature for about 1 hour, until the topping sets, then serve in squares.