How to Make It
Preheat the oven to 450°. In a heatproof medium bowl, cover the anchos with boiling water and let stand until softened, about 20 minutes. Drain the chiles and pat dry. Discard the stems and seeds, then coarsely chop.
Meanwhile, in a small skillet, heat 2 tablespoons of the olive oil until shimmering. Add the pine nuts and cook over moderately high heat, stirring constantly, until lightly browned, about 3 minutes. Add the garlic, cumin and coriander and cook, stirring, until the garlic is softened and the pine nuts are deep golden, about 1 minute longer. Let cool.
In a food processor, puree the pine nut mixture with the anchos and butter. Season the ancho butter with salt and scrape into a bowl.
In a large bowl, toss the broccoli with the remaining 2 tablespoons of olive oil, then spread on 2 large rimmed baking sheets. Roast the broccoli in the upper and lower thirds of the oven for 15 minutes, or until crisp-tender, shifting the pans from top to bottom and back to front halfway through. Add the ancho butter to the broccoli and toss to coat. Season with salt and roast for 10 minutes longer, or until the broccoli is tender. Transfer to a platter and serve warm or at room temperature.
Make Ahead: The recipe can be prepared through Step 3 and refrigerated for up to 3 days.