Combine the butter, brown sugar, corn syrup, salt, and lemon juice in a small, heavy saucepan over medium-high heat. Bring to a boil, stirring often. Once mixture is at a boil, stir constantly for 2 minutes. Remove from heat and sift in the flour a little at a time, stirring to combine each addition before adding the next. Cool to room temperature or refrigerate for later use. If refrigerated, allow mixture to come up to room temperature before cooking.
Heat oven to 350ºF. Line a half-sheet pan with parchment paper and spoon out mixture with a metal soup spoon or a number 50 disher. The candy will spread out like crazy, so place dollops 6 to 8 inches apart. Bake until deep mahogany brown, about 18 minutes.
To form into cups, use a metal spatula to transfer the hot candy disks to the bottoms of inverted ramekins or custard cups — have a ramekin for each disk, or the disks will set before you can get through them all. Work quickly, and the disks will conform to the shape of the cups. Remove when cool and store wrapped with paper towel inside resealable plastic bags.