Preheat oven to 350F. Spray a standard 12-count muffin panwith floured cooking spray, or grease and flour the pan; set aside. For cosmetic reasons, I prefer to avoid the ruffled edges that paper liners create.
In a large mixing bowl, whisk together the the first six dry ingredients, ending with optional salt.
In a separate medium bowl, whisk together the next 6 wet ingredients, ending with vanilla extract.
Pour the wet ingredients over the dry ingredients, and fold to incorporate using a rubber spatula. Don’t stir vigorously or beat; just make sure everything is combined and there are no unblended dry ingredients hiding at the bottom of the bowl or in the folds of the batter. Don’t overmix or muffins will be tough. Batter will be quite thick.
Using a 2-tablespoon cookie scoop or spoon that’s been sprayed with cooking spray (helps batter to slide right off), fill muffin wells with about 2 heaping tablespoons of batter each, coming about one-third of the way full. You want enough of a layer of batter that the jam that’s added in the next step has a nice cushion to rest on so it won’t bottom-out and sink while baking. Re-spray scoop with cooking spray every third or fourth scoopful of batter, or as necessary.
Add 1 to 1 1/2 teaspoons jelly to the center of each cavity (marble-sized mounds)
Top each muffin with remaining batter, about 2 tablespoons each. Fill muffin cups up to a solid 3/4-full, nearly filling to the top. This creates domed, high-top muffins. Underfilled cups create flatter muffins, and you’ll also end up with extra batter.
Bake for 20 to 25 minutes, or until tops are golden, set, domed, and springy to the touch; don’t overbake.
Allow muffins to cool in the pan on top of a wire rack for 5 minutes before removing them from the pan. Then place on a wire rack to cool completely.
Tip: If you’ve accidentally overbaked your muffins, wrapping them while still warm in plasticwrap, and putting them inside a large Ziplock, and sealing it, will help muffins soften over the next day. Muffins are best fresh, but well-wrapped, they will keep airtight at room temperature for up to 5 days, or in the freezer for up to 4 months.