Heat oven to 350 degrees. Butter 6 (8-ounce) ramekins. Place them in any large baking pan where they can sit without touching one another, such as a 9-by-13-inch pan.
In a large bowl, whisk together butter, egg yolks, 3/4 cup sugar, buttermilk, lemon zest and juice, and salt until smooth. Whisk in the flour until well combined.
Using an electric mixer (either handheld or a stand mixer fitted with the whisk attachment), beat egg whites on medium speed until they are thick, white and very foamy, 2 to 4 minutes. Increase speed to high and gradually beat in remaining 1/4 cup sugar, continuing to beat until stiff and glossy peaks form, about 2 to 3 minutes longer.
Using a rubber spatula, gently fold about a third of the egg whites into the lemon mixture, taking care not to deflate the whites. When no white streaks remain, fold in remaining whites, working carefully. Stop folding when most of the second batch of whites are mixed in, but a few white streaks remain.
Spoon the batter into the ramekins. Transfer the pan of ramekins to the oven. Carefully, so you don’t splash the cakes, pour enough very hot tap water into the pan to come about halfway up the sides of the ramekins. The higher the water level, the more pudding and less cake you’ll have.
Bake until the puddings are puffed and lightly browned on top, 35 to 40 minutes. Take care not to overbake, or there won’t be any pudding at the bottom of the ramekins. Carefully transfer the ramekins from the pan to a rack to cool for at least 15 minutes before serving. (The combination of tongs or a spatula, plus oven mitts, works well for moving the cakes.)
As the cakes bake, mix the raspberries and a few pinches of sugar in a bowl, mashing the raspberries slightly to release their juices. Let the raspberries macerate until the sugar melts, about 15 minutes. Then, taste and stir in more sugar if the berries are very tart.
Serve the cakes warm or at room temperature, with the sugared berries and some of their syrup.