Butter a 12- by 17-in. rimmed baking pan. Measure out vanilla and set aside.
In a 2- to 3-qt. saucepan over medium-low heat, stir sugar, butter, 1/4 cup water, corn syrup, and espresso powder until butter is melted and you no longer see sugar crystals. Increase heat to high and boil, stirring occasionally, until mixture reaches 300° on a candy thermometer, 4 to 5 minutes. Quickly remove from heat and carefully stir in vanilla (mixture will bubble up). Immediately spread thinly in prepared baking pan and let cool until hard, about 20 minutes.
Flex pan to release toffee. If using for topping cake, finely chop enough to make 1 1/4 cups. Break remaining toffee into small chunks and reserve for garnish. Store in an airtight container at room temperature for up to 1 week.
Note: Nutritional analysis is per serving.