In a large bowl, with a mixer on medium speed, beat 1/2 cup butter, sugar, and vanilla until smooth. Stir in 1 1/4 cups flour, then beat until dough comes together.
Divide dough into thirds. On a floured surface, with the palms of your hands, roll each portion into a 9-inch-long rope about 1 inch thick. Place ropes 3 inches apart on a buttered 12- by 15-inch baking sheet. Press your finger into dough to make 1/2-inch-wide indentations at 1-inch intervals along each rope. Spoon 1/4 teaspoon jam into each indentation.
Bake ropes in a 350° regular or convection oven until edges are lightly browned, 12 to 15 minutes. Let cool on baking sheet.
Place white chocolate in a plastic sandwich bag, pushing to one corner; secure bag just above chocolate with a twist-tie or knot. Immerse corner of bag in a cup of hot water until chocolate is melted. Dry bag, then, with scissors, cut off the tip of the corner. Squeeze bag to drizzle white chocolate decoratively across ropes. Chill until chocolate is firm to touch, about 1 hour, then cut each rope diagonally into 12 slices.
Nutritional analysis per cookie.