Preheat the oven to 350℉ (180℃). Lightly grease a 9x9-inch (23x23 cm) square baking pan.
Cream the butter and sugar until they are smooth and fluffy. Add the flour and mix until they form small, even crumbs. Press the dough into the bottom of the prepared baking pan.
Bake the crust for 10 minutes.
While the crust is baking, make the filling - whisk together the eggs, brown sugar, melted butter, vinegar, and vanilla. Stir in the chopped pecans.
Remove the partially baked crust from the oven, and press down any air bubbles with the back of a spoon. Pour the filling mixture over the hot crust and return it to the oven.
Bake for a further 20-25 minutes, or until the filling is set at the edges and still a bit jiggly in the center. Remove the pan from the oven and let the bars cool for 10 minutes. Run a butter knife or thin spatula around the inside edges of the pan to loosen the filling from the pan, so that it doesn't stick to the pan as it cools.
Allow the bars to cool in the pan and the filling to set completely (3 - 4 hours) before cutting them into 9 squares, then cutting each square diagonally into 2 triangles.
Makes 18 triangle bars.
Notes
Try other soft, buttery nuts - walnuts or macadamia nuts would be delicious.Or replace the pecans with 1 cup of raisins or currants to make Butter Tart Bars.