PREP: Preheat oven to 400 degrees with a rack in the upper third. Line a baking sheet with parchment paper.
MIX DRY INGREDIENTS/CUT IN BUTTER: In the bowl of a food processor fitted with a steel blade (or in a large bowl using a pastry cutter or your fingers), measure in the flour, cornmeal, brown sugar, baking powder, and salt. Pulse/stir a few times to mix. Add the pieces of butter and pulse/cut in/rub into the flour carefully until you have pea-sized and smaller pieces of butter.
TURN OUT FLOUR MIXTURE INTO A LARGE BOWL (if using the food processor/if you're using a pastry cutter or your fingers, the mixture will already be in the bowl) and stir in the diced plums. In a measuring cup, whisk together the sour cream or yogurt, milk, and almond extract, and pour that into the bowl. Using a table fork, stir until all the dry bits are incorporated, but the dough is still shaggy. Knead a few times until the dough barely holds together, then turn it out onto a lightly floured counter or board. The dough shouldn’t be uniform, smooth, or neat. Do not overwork the dough. (If by chance the dough is too sticky to work --and it shouldn't be if you've patted the fruit with paper towels -- sprinkle with another tablespoon of flour.)
PAT DOUGH with floured hands into a 1-inch-thick rectangle approximately 6” x 10”. Brush with lightly with about a tablespoon of the egg mixture and sprinkle with a little granulated sugar— 1 or 2 teaspoons. (You will have egg leftover you can cook for yourself or the dog.) Sprinkle the sliced almonds evenly on top and pat gently into the dough. Using a bench scraper or a knife, cut into first 2, then 4, then 8 (or then 16 for small scones—chill an hour before baking) rectangles. I like using the bench scraper to move the scones to the sheet pan. Place on prepared sheet pan as far apart as possible.
BAKE scones until golden on top and crispy on the bottom—about 20 minutes. Remove to a rack and cool for a minute or two before serving warm. Scones will keep for a day or two, well-covered but are best the day they're made. To reheat, split in half lengthwise, spread lightly with butter, and grill until toasty and hot in a stove top grill pan or skillet. Wrap well, place in freezer bag, and freeze baked scones for a month only if necessary.