Heat oven to 350°F. Line a 9-inch square pan with parchment, leaving overhang.
Combine flour, sugar, and salt for the crust in a bowl. Cut in cold butter until sandy crumbs form.
Press the mixture firmly into the pan. Bake 12–15 minutes, until set and lightly golden at the edges.
Whisk brown sugar, corn syrup, melted butter, and the 1 tbsp flour until smooth.
Stream in the beaten eggs. Add vanilla, vinegar, and salt. Whisk until glossy and well combined.
Fold in raisins so they’re evenly distributed.
Pour the filling over the hot crust. Smooth the top with a spatula.
Bake 20–22 minutes, until edges are set and the center still jiggles slightly.
Cool completely in the pan. Chill 30 minutes for cleaner cuts, then lift and slice into squares.