In a food processor, add the 1 1/4 cups of flour, sugar, the salt and pulse to combine. Add the butter and pulse until the mixture resembles coarse meal. Add sour cream and the ice water and pulse until the dough just barely comes together. Gather the dough and pat it into a disk. Wrap the dough in plastic and refrigerate until chilled, about 30 minutes.: You will first notice the dry scents of flour and sugar coming together, faint and comforting. Pulsing quickly combines them without warming the mixture, which is important because heat will soften the butter . The little noisy bursts of the processor are normal, and you should stop before a fine powder forms. If you overprocess the mixture it can lead to a tougher crust because of excess gluten development. A common mistake is running the processor too long so the mixture feels warm to the touch, which signals the butter is losing its coldness. If that happens, chill the bowl briefly before continuing.
Preheat the oven to 425℉. Line a baking sheet with parchment paper.: As you add the cold, cubed butter , the texture should change to small, pea sized bits that glint slightly. This visual cue tells you the butter is properly distributed and will create steam pockets while baking. You might hear rapid pulses and see bits of butter the size of peas, that is exactly right. Avoid overworking to the point where the mixture resembles a paste, because that will remove the flakes from the final crust. If the butter seems too soft, pop the bowl into the refrigerator for five to ten minutes and then continue.
Working on a lightly floured surface, roll out the disk of dough to a 12-inch round; transfer to the baking sheet.: The mixture will feel slightly tacky and hold if you pinch it, but it should not be wet or sticky. The cold sour cream and ice water bind the dough while maintaining coolness, which helps the crust stay flaky. When you squeeze a small handful it should cling without oozing moisture. Overmixing at this stage produces a dense crust, so pulse just enough for cohesion. If your dough separates or falls apart, add another teaspoon of ice water and pulse gently.
In a bowl stir the brown sugar with the cinnamon together.: Touching the dough gives you immediate feedback, it should be cool and slightly firm. Patting it into a flat disk ensures even chilling and easier rolling later. If the dough feels warm, the butter may have softened too much; return it to the refrigerator until firm. A common error is rolling a warm dough which will stick and tear, so chilling is not optional for a neat crust.
Arrange the plums in the center of the dough, leaving a 1 1/2-inch border all around. Sprinkle with the sugar mix evenly over the plums. Fold the edge of the dough up and over the plums. Brush the rim with the egg wash and sprinkle with additional brown sugar if preferred.: Chilling lets the butter firm up and the flour hydrate, which reduces shrinkage during baking. You will feel the dough go from pliable to slightly firm after resting. If you skip this step you risk a loose, shrinking crust that loses structure. If you are short on time you can rest it in the freezer for about 15 minutes, but watch it closely to avoid freezing solid.
Bake the galette for about 45 minutes, or until the crust is golden and the fruit is tender and bubbling. Let the galette cool on the baking sheet for 30 minutes, sprinkle with some powdered sugar, then cut into wedges and serve.: The oven will radiate dry heat and the initial high temperature encourages a brisk rise and browning. The scent shift from kitchen to bakery begins here, with a warm, toasty smell as the oven comes up to temperature. Using parchment prevents the filling from sticking and helps with even browning. A common oversight is putting the galette into an oven that has not fully reached temperature, which can yield a soggy bottom rather than a crisp crust.
Working on a lightly floured surface, roll out the disk of dough to a 12-inch round; transfer to the baking sheet: Rolling should produce a slightly imperfect round that is about twelve inches across. You will see layers and a powdered dusting of flour to prevent sticking. Keep the dough moving and rotate as you roll to maintain an even thickness. If you press too hard you will compress the layers and get a dense crust, so use gentle, even pressure. If the edge cracks, simply press it back together with your fingers and keep a light dusting of flour handy.
In a bowl stir the brown sugar with the cinnamon together: The dry aroma of cinnamon releases as you mix it with the brown sugar , and the combination will smell warmly spiced. This mixture creates the caramelizing element for the fruit, so get an even blend to avoid clumps. If the cinnamon is unevenly distributed you may get pockets of spice that overwhelm a bite, so whisk until uniform.
Arrange the plums in the center of the dough, leaving a 1 1/2-inch border all around. Sprinkle with the sugar mix evenly over the plums. Fold the edge of the dough up and over the plums. Brush the rim with the egg wash and sprinkle with additional brown sugar if preferred: When you layer the plums they should overlap slightly and create a textured, jewel like center. As the oven heat hits them their juices will begin to glisten and smell fruity and caramel like. Folding the edge up creates a rustic frame that contains juices while exposing the fruit top to direct heat, encouraging caramelization. Brushing the rim with the egg yolk wash adds a glossy, deeply golden finish. A common slip is piling the fruit too high which traps steam and prevents proper caramelization, so keep a single generous layer.
Bake the galette for about 45 minutes, or until the crust is golden and the fruit is tender and bubbling. Let the galette cool on the baking sheet for 30 minutes, sprinkle with some powdered sugar, then cut into wedges and serve: In the oven you will hear faint bubbling and smell the sugar caramelizing, with the crust taking on an even, warm brown. The sound of gentle bubbling signals that the juices are thickening and the plums are tender. Letting it cool on the sheet allows the filling to set so slices hold shape instead of running. A mistake is slicing too soon, which yields a messy plate, so be patient and wait until the fruit has settled before cutting.