Butter a 9x9 inch baking dish or similar-sized dish, coating the bottom and sides generously. This prevents the caramel from sticking and makes it easy to remove once set. Set aside while you prepare the caramel mixture.

In a microwave-safe bowl (I recommend glass or ceramic, not plastic, as the mixture gets very hot), combine the butter, brown sugar, sugar, corn syrup, and sweetened condensed milk. Stir everything together until well combined—the mixture should look somewhat uniform, though the sugars may still be grainy. This initial mixing ensures even cooking in the microwave and prevents hot spots.

Microwave the mixture on high for 6 minutes without stirring. The caramel will bubble up and darken as it cooks—this is exactly what you want. At 6 minutes, carefully remove the bowl (use oven mitts, as it will be extremely hot) and test the consistency by dropping a small spoonful into ice water. If it forms a soft ball that's pliable but holds its shape, it's done. If it's too soft, return it to the microwave for 30 seconds and test again.

Once the caramel reaches the correct consistency, stir in the vanilla extract immediately—I add it at the end to preserve its delicate flavor notes rather than letting them cook off during the microwave time. Carefully pour the hot caramel into the buttered baking dish, working quickly but safely. The caramel will continue to set as it cools, so don't worry if it seems slightly softer than you'd like—it firms up considerably once it reaches room temperature.

Let the caramel cool at room temperature for at least 30 minutes until it's completely set and no longer sticky to the touch. Once set, cut into squares using a buttered or oiled knife (this prevents sticking). Store in an airtight container at room temperature for up to 2 weeks, or wrap individual pieces in parchment paper for easy serving.