Combine the sugar, corn syrup and ½ cup water in an 2-quart glass measure. Cook, uncovered, at 100% for 3 minutes.

Remove from the oven and stir thoroughly. Add the peanuts; stir again. Cover tightly with microwave plastic wrap. Cook at 100% for 15 minutes.

Lightly coat a spatula and a large baking sheet or a 16 × 12-inch marble slab with vegetable oil. Remove the syrup from the oven. Pierce the plastic to release the steam and uncover carefully. Pour the mixture onto the oiled surface. With the oiled spatula, spread the peanuts to distribute them evenly through the cooling syrup. Let cool and harden.

When the brittle is cool, break into chunks with a wooden mallet or rolling pin. Store in an airtight container.