Line an 8×8-inch baking dish with parchment paper, leaving a little overhang on the sides. Lightly butter the parchment and set aside.
In a large microwave-safe bowl, melt the butter. This usually takes about 30–45 seconds.
Stir in the granulated sugar, brown sugar, corn syrup, sweetened condensed milk, and salt until well combined.
Microwave the mixture uncovered for 6–8 minutes total, stopping every 2 minutes to carefully remove the bowl and stir. The caramel will bubble up, so make sure your bowl is large enough.
The caramel is ready when it turns a deep golden color and thickens slightly. If it still seems thin, microwave in additional 30-second increments, stirring each time.
Carefully stir in the vanilla extract.
Pour the caramel into the prepared pan. Let it cool at room temperature for 2–3 hours, or until fully set.