ATK found that combining cocoa powder, Dutch-processed for the richest color, and semisweet chocolate produced the fudgiest flavor.
Makes about 2 cups
10 ounces semisweet chocolate, chopped
1/3 cup Dutch-processed cocoa powder, sifted
3/4 cup light corn syrup
1/3 cup sugar
1/3 cup heavy cream
1/3 cup water
Pinch table salt
3 tablespoons unsalted butter, cut into 1/4-inch pieces
1 teaspoon vanilla extract
1. Microwave chocolate, whisking often, until melted and smooth, 1 to 3 minutes. Whisk in cocoa until dissolved.
2. Meanwhile, simmer corn syrup, sugar, cream, water, and salt in medium saucepan over medium heat, stirring frequently, until thickened, about 4 minutes.
3. Off heat, whisk in butter and vanilla. Cool mixture slightly, about 2 minutes, before whisking in melted chocolate until smooth. Serve warm.
The sauce can be refrigerated in an airtight container for up to 10 days. Reheat in a small saucepan over low heat or microwave, stirring often, until warm and smooth, 1 to 3 minutes.