Have the oven broiler on or a grill heated to medium-high.
Peel the mangoes with a paring knife. One at a time, set each mango on its side on a cutting board and cut the flesh away from the large flat seed in the center. Place the largest 2 pieces from each mango in a medium bowl (set the rest of the flesh aside for another use).
Melt the butter in a small skillet over medium heat and cook until it turns nut brown. Add the Toasted Spice Rub and salt. Remove the skillet from the heat. Pour or brush the butter mixture over the mangoes and toss well to coat them.
Grill the mango slices (only on the flat, cut side) over medium-high heat until nicely marked, 3 to 5 minutes.
While mangoes are grilling, cut the blue cheese into 1-inch pieces.
Remove the mango slices from the grill and spread them on a baking sheet, with the grilled sides facing up. Divide the cheese on top of the mango slices.
Place under the broiler for 30 seconds to weep the cheese (optional step). Be careful not to get the mango too hot. Arrange on a platter or individual plates.
In a heat-proof bowl, warm the honey briefly in the microwave and drizzle over the top. Serve warm.
Toasted Spice Rub:
1/4 cup fennel seeds
1 tablespoon coriander seeds
1 tablespoon peppercorns
1 1/2 teaspoons red pepper flakes
1/4 cup pure California chili powder
2 tablespoons kosher salt
2 tablespoons ground cinnamon
Toast the fennel seeds, coriander seeds, and peppercorns in a small, heavy pan over medium heat. When the fennel turns light brown, work quickly. Turn on the exhaust fan; add the red pepper flakes, and toss, toss, toss, always under the fan. Immediately turn the spice mixture out onto a plate to cool. Put in a blender with the chili powder, salt, and cinnamon and blend until the spices are evenly ground. If you have a small spice mill or a coffee grinder dedicated to grinding spices, grind only the fennel, coriander, pepper, and chili flakes. Pour into a bowl and toss with the remaining ingredients.
Toasting freshens spices, releases their oils, and makes them more fragrant, as well as adding a new dimension of flavor. Keep the spice mix in a glass jar in a cool, dry place, or freeze.
Taste your chili powder and, if spicy and hot, cut back the amount. California chiles are almost sweet, not hot.