Mix the saffron and milk together in a large bowl and let stand for 20 minutes. Then mix in ½ cup sugar and the yeast. Let it stand in a warm place until the yeast bubbles. Mix in the eggs, salt, and flour. Knead in the butter a few cubes at a time until all the butter is incorporated. This is best done with an electric mixer that has a dough hook, because it is important that the dough not get too warm. Add the pistachios with the last of the butter.
Cover the dough with plastic wrap and refrigerate until it is completely chilled, about 2 hours.
Butter and dust with flour a coffee or large juice tin. With lightly floured hands, shape the dough and place it in the tin, which should be no more than two-thirds full. Let the dough rise until it reaches the top of the tin.
Heat the oven to 350°F. Bake the brioche until it is dark golden and sounds hollow when thumped, about 1 hour. Let it cool and then remove from the tin.
While the brioche is rising, put the remaining ½ cup sugar and the water in a saucepan and bring to a boil. Add the pears and add more water if necessary to cover them. Simmer until the pears are soft and begin to break apart, about 1 hour. Add more water if necessary because the pears will absorb it. Let the pears cool, then push them through a sieve or fine food mill. Strain as often as necessary to get a fine sauce. Mix in the Chartreuse and then brush the glaze over the brioche. Serve each slice of brioche with chantilly cream.