Set oven rack about 6 inches from the heat source and preheat the oven's broiler. Place lemon halves, cut-side up, on a baking sheet.
Broil until lemon halves are blackened on the cut side, about 10 minutes. Allow lemons to cool enough to handle. Squeeze juice from the lemons into a measuring cup. Measure out 1 fluid ounce broiled lemon juice, reserving the rest for another use.
Combine whiskey, orange liqueur, simple syrup, and 1 fluid ounce broiled lemon juice in a blender and add ice; cover and blend until slushy consistency. Pour drinks into glasses.