A combination of tender shortcake biscuit, fresh summer fruit, and a lightly spiked whipped cream makes for a very adult, very luscious dessert.
Slice 1/3 of the plums and put into a small pan on the stove. Add 3 tablespoons of the sugar and the water and cook on low until the fruit starts to soften, about 5 minutes.
Remove from the heat, add 2 tablespoons of the dessert wine, and continue to simmer the mixture until the syrup thickens, about 1-2 minutes. Taste and add 1 additional tablespoon of sugar, if necessary. Strain the plum syrup through a strainer and discard the solids. Cool.
Cut the remaining plums in half and remove the pits. Slice thinly and toss in a bowl with the remaining dessert wine and 1 tablespoon of sugar. Set aside.
In a large, chilled metal mixing bowl, whip the heavy cream, sugar, and sweet wine, if using, with an electric mixer on low until well blended. Increase the speed to medium-high and whip until soft peaks form. Use immediately or refrigerate, covered, until ready to serve.
Position a rack in the center of the oven and preheat the oven to 425 degrees.
Line a large, heavy-duty baking sheet with parchment paper. Combine the flour, sugar, baking powder, baking soda, and salt in the bowl of a food processor and pulse a couple of times to mix. Add the butter and pulse until the mixture resembles coarse cornmeal.
In a small bowl, beat the egg and ¼ cup of the heavy cream with a fork and mix in the buttermilk. Add to the dough in the processor through the feed tube as you pulse until it all comes together (it might still look a little shaggy).
Lightly dust a work surface with flour. Remove the dough from the processor and gently knead it 3 or 4 times on a floured surface. If the dough seems dry and doesn’t form a cohesive mass, work in more cream, 1 teaspoon at a time.
Roll the dough into a ¾-inch-thick disk. With a sharp 2 ½-inch biscuit cutter, press straight down to cut the dough into rounds, and lift straight up to remove (don’t twist the cutter or it will seal the sides of the biscuits and interfere with rising).
Transfer the rounds to the baking sheet. Gather the dough scraps, gently knead them together, re-roll, and cut out more biscuits until you have a total of 6.
Lightly brush the biscuit tops using 1 tablespoon heavy cream and sprinkle with the turbinado sugar. Bake, rotating the baking sheet once, until the biscuit tops are lightly browned, 10-15 minutes. Let the biscuits cool slightly.
Using a serrated knife, split the warm biscuits in half horizontally and transfer the bottoms to 6 dessert plates. Spoon about three-quarters of the macerated plums and the plum sauce evenly over the biscuit bottoms. (It’s OK if some of the fruit spills out onto the plate.)
Top with a generous dollop of whipped cream and cover each with a biscuit top. Spoon more plums and cream over each shortcake. Sprinkle with toasted almonds and serve immediately.