Liqueur di Marmellata di Pompelmo

I tried making scorched orange halves in sugar over the BBQ and forgot about them, leaving the sugar to caramelize very darkly and mercilessly stick the fruit to the pan. I figured I'd dissolve it by adding vodka, and a day later they'd come free. I let them settle and the taste was great: citrusy, dark, complex, amaro-like.  It tasted like an old fashioned liqueur you'd find in Italy, so I've come to think of it as "nona's liqueur".  I've also made it, the same way, with lemon and had the same tasty results.

This time, I'm trying on the stovetop with grapefruit, measuring the quantities to see if I can reproduce it reliably.

450 g Grapefruit (1 whole), halved, sliced
100 g Sugar
750 ml Vodka, clean tasting (I used Frïs here)

Sprinkle half the Sugar in a cast iron pan.
Lay in the sliced Grapefruit.
Add the remaining Sugar.
Let sit to absorb Sugar and draw out the juice.
Heat high and cook down, and begin to caramelize the syrup.
Continue, turning Grapefruit until it gets as dark as you can stand, it will start smoking.
Take off heat and cool.
Pour on Vodka.
Let sit to dissolve baked-on caramel.
Pour into jar and let mellow until your curiosity persuades you to drink it over an ice cube.