Grease and line an 8×8-inch square pan with parchment, making sure that the parchment runs up the sides. I like to use metal 8x8-inch square pans like this one on Amazon. Open the liquid pectin packets and stand them up in a mug or a glass by the stove for later.
Bring a medium pot of water to boil, and boil the whole grapefruit for about an hour to ensure the peel has softened. Let cool slightly, then cut the boiled grapefruit in half. You will only need one half for a batch of pâte de fruit—I suggest you make a quick jam with the other half.
Cut the boiled grapefruit half in large chunks, removing any pits along the way. Place the grapefruit in a food processor and blend for at least a full minute to completely purée the fruit and to chop all the fibers/skin.
Transfer the fruit purée to a large pot, along with the juice, honey, and granulated sugar. Bring the mixture to a boil on high, stirring constantly. Monitor the temperature with a digital candy thermometer (like this probe thermometer from Thermoworks: the DOT).
When the mixture hits 118ºF (48ºC), add the butter, and continue to stir and cook the mixture until it hits 223—224ºF (106ºC).
When you reach the final cooking temperature, immediately take the pot off the stove and dump in the liquid pectin. Stir well to make sure it gets evenly mixed in, then transfer the hot mixture to the prepared pan.
Let the pâte de fruit set overnight, uncovered, until it is completely set throughout. Unmold the pâte de fruit, and cut it into 1-inch squares with a large, oiled kitchen knife and, just before serving, toss the cubes in granulated sugar to coat them. Store the uncoated pâte de fruit between parchment sheets.