Prep: Preheat oven to 350°F (175°C). Line a 9x9 inch baking pan with parchment paper.
Crust: Mix the crust ingredients (flour, icing sugar, cold butter) in a food processor until crumbly. Press into the pan.
Pre-bake: Bake the crust for 5–15 minutes until lightly golden.
Filling: Whisk brown sugar, eggs, melted butter, flour, vinegar, and vanilla until smooth.
Assemble & Bake: Pour filling over the hot crust. Bake for 35–40 minutes until set and bubbly.
Cool: Cool completely before cutting, as this allows the ooey-gooey center to set.