Several hours ahead or the day before, separate the rose petals, rinse them quickly in cold water and pat them dry. Put one egg white in a small bowl and whisk until it's foamy. (You may or may not need the second white.) Put the sugar in another small bowl and place a sheet of parchment paper or a silicone baking mat on the counter. One at a time, dip a petal into the white and let the excess drip back into the bowl. Drag the petal through the sugar to coat both sides very lightly. Dry the petals on the paper or mat in a cool, non-humid place for at least 6 hours or for as long as overnight.
Coarsely chop the berries and toss them into a small saucepan with the sugar. Put the pan over medium heat and cook, stirring, for 5 to 8 minutes, or until the juices are slightly thickened and syrupy. Scrape the berries and syrup into a bowl, stir in the rose extract and cool to room temperature. (You can make the compote up to 3 days ahead and keep it covered in the refrigerator.)
Center a rack in the oven and preheat the oven to 425°F. Line a baking sheet with parchment paper or a silicone baking mat.
Put the sugar and lemon zest in a large bowl and, working with your fingertips, rub the ingredients together until the sugar is moist and fragrant. Add the rest of the dry ingredients to the bowl and whisk to combine. Drop in the pieces of cold butter and, again using your fingertips, crush, rub and blend the butter in. You'll have flakes of butter and small pieces and this is just right. Pour the cold buttermilk over the mixture, switch to a fork and toss and stir everything together until the milk is absorbed - your dough might look like curds, but that's fine. Don't stir too much, too vigorously or for too long and if there are a few dry spots in the bottom of the bowl, ignore them. Reach into the bowl and knead the dough gently, folding it over on itself and turning it over 6 to 8 times.
Dust a work surface lightly with flour, turn out the dough and, still using your hands, pat the dough out until it is 1/2 inch thick. (The thickness is what's important here.) Using a high-sided 2 inch cutter, cut out biscuits and place them on the baking sheet. Pat the scraps together until they're 1/2 inch thick and cut out as many more biscuits as you can. (The leftover dough can be cut into biscuits, but they won't rise as high or as evenly as the others - you can keep them as your baker's treat). Sprinkle tops with sugar.
Bake for 15 to18 minutes, or until the biscuits have risen gloriously and their tops and bottoms are golden brown. Transfer the baking sheet to a cooling rack and allow the biscuits to cool until they reach room temperature. (The biscuits can be made up to 6 hours ahead; keep them uncovered at room temperature.)
Working with an electric mixer, beat the cream just until it mounds softly. Still beating, add the sugar, followed by the vanilla and rose extracts. When the cream is fully whipped and holds firm peaks, quickly beat in the sour cream, if you're using it. To tint the cream, beat in just one drop of coloring; continue adding coloring a tiny drop at a time until you get the shade of pink you want. (The whipped cream can be made up to 3 hours ahead and kept tightly covered in the refrigerator; whisk a couple of times before using.)
Just before you're ready to put the shortcakes together, stand the berries up and, using a thin-bladed knife, cut each berry into 4 or 5 thin slices.
If you'd like to pipe the whipped cream, either spoon the cream into a pastry bag fitted with an open star or plain tip, or spoon the cream into a zipper-lock plastic bag and snip off a corner. Alternatively, you can simply spoon on the cream.
Slice off the top of the biscuit to create an even surface for piping the cream. Save the tops to nibble on later. Put a teaspoonful of strawberry compote and syrup in the center of each biscuit. Pipe (or spoon) a circle of whipped cream around the compote, leaving a bit of compote uncovered. Finish each shortcake by pressing two or three slices of strawberry together, fanning them out a little and placing them, broad side down, in the center of each cake. Add a rose petal for the finishing touch. (If you have any extra compote and/or cream, cover and keep in the refrigerator to enjoy at another time.)
Arrange the shortcakes on a platter. Scatter the remaining rose petals around the platter and serve immediately.
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