In a microwave-safe bowl, heat 1½ bars (6 ounces) chocolate on high in 30-second intervals, stirring between each, until chocolate is melted and smooth (about 1½ minutes total). Stir in chocolate syrup until smooth.
In a large bowl, beat butter and sugar with a mixer at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
In a medium bowl, whisk together flour, salt, and baking powder. With mixer on low speed, gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition. Roughly chop remaining ½ bar (2 ounces) chocolate; stir chopped chocolate, melted chocolate mixture, and cherries into batter. Spoon into prepared pan.
Bake until a wooden pick inserted near center comes out clean, about 1 hour and 30 minutes, loosely covering with foil to prevent excess browning, if necessary. Let cool in pan for 15 minutes. Remove from pan and let cool completely on a wire rack.