(While the chiles are hydrating in hot water, you’ll char the peppers and toast the almonds in the oven.)
In a dry pan/skillet over medium-high heat, toast the chiles until they are very fragrant and you just begin to see a bit of smoke rising from the pan. Pull the pan off the heat and place the chiles in a small bowl or 1-cup glass measuring cup.
Pour the hot water over the chiles. Allow them to sit at room temperature while you continue with the recipe. (I will use a spoon or a very small bowl to submerge the chiles.)
Preheat the oven to high broil. Set the top rack about 6” below the coils. Line a small, rimmed baking sheet with foil, or you can use a small baking dish, like an 8x8”.
Place the peppers on the baking sheet and broil for 2 to 4 minutes, or until the top is charred. Using tongs, carefully rotate the peppers and continue to char until blackened all over. Your oven will be very hot. Be careful.
Once both peppers are blackened all over, remove the pan from the oven. Carefully wrap the foil from the baking sheet around the peppers and allow them to sit for 5 to 10 minutes.
Turn the oven to 375 degrees F.
Unwrap the foil and allow the peppers to cool until you can handle them and pull the charred skin off, discarding the skin. Open the peppers and remove the seeds and stem and discard. (See Notes on making the peppers in advance.) Place the peppers in a blender.
Place the almonds on the same baking sheet you used for the peppers. You may line it with parchment paper, or just use the clean baking sheet.
Toast the almonds for 8 minutes. Stir and toast again for 5 to 6 minutes, or until they are very fragrant and lightly browned. (Don’t over-brown them or they will be bitter-tasting.) Pour them onto a clean plate or cutting board to cool.
Your chiles should be nicely pliable by now. Using clean kitchen scissors or your hands (disposable gloves help here!), cut open the chiles, remove the seeds and stems and discard. Place the chiles in the blender with the peppers. Strain the chile’s liquid through cheesecloth to remove any grit that may be left in the bowl. Set the liquid aside; do not discard it.
Add the garlic, tomato paste, parsley, paprika, red wine vinegar, cayenne and red pepper flakes if using, and the oil to the blender.
Add the cooled almonds.
Puree the mixture until it is the consistency you prefer, adding small amounts of either the strained chile liquid or more oil, or a combination of both, as needed for the sauce to be able to flow in the blender. Some prefer a very smooth velvety sauce, others prefer some texture. You will be stopping and scraping often. A silicone spatula is helpful here.
Set the mixture aside if you are going to make the roasted cauliflower now. Cover the sauce well and refrigerate if you are making the cauliflower at a later time. The sauce can be refrigerated for up to 4 days.