Preheat oven to 350F.
Grease well and flour two 8-by-l-l/2-inch layer-cake pans, or a 13-by-9-by- 2-inch baking pan.
Melt chocolate over hot, not boiling, water; let cool.
Into large bowl of electric mixer, sift flour with sugar, soda, and salt.
Add shortening and sour cream.
At medium speed, beat 2 minutes, occasionally scraping side of bowl and guiding mixture into beaters with rub- ber scraper.
Add eggs, vanilla, chocolate, and 1/4 cup hot water; beat 2 minutes longer.
Pour batter into prepared pans; bake layers 30 to 35 minutes; oblong 35 to 40 minutes; or until surface springs back when gently pressed with fingertip.
Cool in pans 10 minutes.
Remove from pans; cool thoroughly on wire racks.
Fill and frost as desired.