Cook sugar in a dry 1-quart heavy saucepan over moderately high heat, undisturbed, until it begins to melt, about 2 minutes. Continue to cook, stirring occasionally with a fork, until sugar is melted into a deep golden caramel, 1 to 2 minutes.
Remove from heat and carefully pour in cream (mixture will steam and bubble vigorously). Once bubbles begin to subside, return pan to moderate heat and cook, stirring constantly, until caramel is dissolved.
Remove from heat, then add chocolate and salt and stir until chocolate is melted. Add butter and stir until just melted.
Cool sauce slightly, then drizzle over strawberries.