1. Combine sugar, water and agave nectar in a heavy medium saucepan; stir well. Cover, and boil until sugar has dissolved (about 5 minutes), stirring once or twice. Cover again, and boil until bubbles are large and mixture is thick (4 minutes). Uncover, and continue boiling 6 minutes or until mixture is golden.
2. Remove pan from heat; place bottom of hot pan in cold water. Carefully add the butter, vanilla, and salt (the mixture will bubble and hiss); stir with wooden spoon. Stir in lemon juice and espresso powder mixture. To use right away, let the sauce cool slightly to thicken. For later use, let stand at room temperature for up to one day. If the sauce hardens, warm in microwave until soft (about 15-25 seconds).
3. To serve: Fill a waffle cone with 1/2 cup frozen yogurt. Drizzle 1 tablespoon sauce over the frozen yogurt; top with 1 teaspoon toasted almonds. Repeat with the remaining cones. Store leftover sauce, covered, in the refrigerator for up to a week.