How to Make It
Bring sugar and water to a boil in a small saucepan over medium-high. Cook, stirring often, until sugar completely dissolves, about 3 minutes. Remove from heat. Stir in lemon zest and juice; chill, uncovered, 1 hour.
Cut cake into 8 (1 1/4 -inch-thick) slices. Spread butter over cake slices. Preheat grill to medium-high (400°F to 450°F). Place cake slices on grill rack coated with cooking spray. Grill, uncovered, until grill marks appear, about 3 minutes per side. Serve slices on shallow plates; drizzle with lemon syrup; dollop with whipped topping.