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Chocolate is derived from the roasted seeds of Theobroma cacao . The toxic principles in chocolate are a group called methylxanthines which includes theobromine and caffeine. Although the concentration of theobromine in chocolate is 3-10 times that of caffeine, both constituents contribute to the clinical syndrome seen in chocolate poisoning. The exact amount of methylxanthines in chocolate varies due to natural variation of cocoa beans and variation within brands of chocolate products. However, in general, the total methylxanthine concentration of dry cocoa powder is ~800 mg/oz (28.5 mg/g), unsweetened (baker’s) chocolate is ~450 mg/oz (16 mg/g), semisweet chocolate and sweet dark chocolate is ~150-160 mg/oz (5.4-5.7 mg/g), and milk chocolate is ~64 mg/oz (2.3 mg/g). White chocolate is an insignificant source of methylxanthines. Cocoa bean hulls contain ~255 mg/oz (9.1 mg/g) methylxanthines.
The LD50 (Lethal dose 50) of caffeine and theobromine are reportedly 100-200 mg/kg, but severe signs and deaths may occur at much lower doses and individual sensitivity to methylxanthines varies. In general, mild signs and discomfort like vomitting and drinking lots of water may be seen in dogs ingesting 20 mg/kg, dangerous effects on heart may be seen at 40-50 mg/kg, and seizures and convulsions may occur at doses ≥60 mg/kg.

And for everyone's information,
One ounce of milk chocolate per pound of body weight is a potentially lethal dose in dogs.