Beat 7/8 cup butter and confectioners' sugar together with a spoon in a large bowl until smooth. Stir in flour and salt. Press mixture into a 9-inch square baking pan.
Bake in the preheated oven until lightly golden, about 14 minutes. Remove to cool 10 for minutes, leaving oven on.
Beat eggs, brown sugar, 3 tablespoons melted butter, and vanilla extract together in a bowl. Sprinkle raisins over cooled crust; pour egg mixture over raisins.
Bake in the preheated oven until top springs back when touched, about 30 minutes. Cool completely before cutting into bars.
