Preheat oven to 300. Sift together flour & salt into medium bowl.
Beat butter with mixer on med-high speed until fluffy, 3-5 min,
scraping down sides of bowl with spatula. Add sugar gradually
beating between additions, continue until mixture is very pale,
about 2 min total. On low, add flour mixture & beat until just
combined. After beating flour, beat in 4 oz bittersweet chocolate,
3/4 cup, Press dough evenly into a parchment lined 9x13 rimmed
baking sheet, refrigerate until firm about 20 min. Cut dough in
eights lengthwise, then quarters crosswise making 32 rectangles.
Evenly pierce all over with a skewer.
Bake until shortbread is firm in center and golden brown, 50 min
to 1 hour. Transfer pan to wire rack; immediately recut rectangles
with a paring knife. Let shortbread cool completely in pan.
Melt 5 oz bittersweet chocolate. Dip bars into chocolate & place on
parchment lined baking sheet and refrigerate until chocolate is set,
about 10 min
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