Combine sugar, water, and syrup in a small saucepan, stirring well; cook over medium heat, stirring constantly, until sugar dissolves and mixture comes to a boil. Cover and boil 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches soft ball stage (240°).
Pour syrup onto a marble slab or cookie sheet that has been wiped with a clean, moist cloth. (Do not scrape syrup from pan. ) Cool syrup to lukewarm (110°). Using a well-buttered candy scraper or stiff spatula, scrape syrup together in center mass; spread mass out over the slab using long, firm strokes. Continue scraping together and spreading the syrup mixture 10 minutes or until fondant is white, crumbly, and too stiff to stir. Moisten hands, and gather fondant into a ball, using spatula. Knead fondant with heel of hand 10 minutes or until smooth and free of lumps.
Add peppermint oil, and continue kneading until flavoring is well blended. Shape fondant into 1/2-inch balls; place on waxed paper. Pressing gently, flatten balls to 1/4-inch thickness. Set aside.
Grate 2 squares of chocolate; sift grated chocolate through a fine sieve. Set chocolate aside.
Chop remaining 6 squares chocolate into coarse pieces; place in a stoneware bowl over, but not touching, hot water (115°). Stir with a dry wooden spoon until chocolate melts. Remove bowl from over water, and set on a towel.
Add 1 teaspoon sifted, grated chocolate to melted chocolate; stir until chocolate melts. Repeat procedure until chocolate in bowl cools to dipping temperature (90°).
Using a fork, dip reserved mint patties, one at a time, into melted chocolate, covering each patty completely with chocolate. Tap fork on edge of bowl to remove excess coating. Transfer to a waxed paper-lined baking sheet. (If chocolate becomes too thick, place over hot water until chocolate reaches dipping temperature (90°). Store in airtight containers.