Make cake: Preheat oven to 325°F with rack in middle. Flip bottom of springform pan so lip is facing down, then lock in place. Butter bottom and side of pan, then dust bottom with flour, knocking out excess.
Whisk flour, cocoa, baking powder, baking soda, and salt in a large bowl.
Melt chocolate with butter in a 3-quart heavy saucepan over low heat, stirring. Remove from heat and whisk in eggs and water until smooth. Whisk into flour mixture, then whisk in sugar until smooth. Pour into pan and rap on counter to eliminate any air bubbles.
Bake until cake is just starting to pull away from side of pan and a wooden pick inserted into center comes out clean, 1 1/4 to 1 1/2 hours.
Cool cake in pan 20 minutes, then remove side of pan and cool completely, at least 1 hour.
Make glaze: Melt chocolate in a small metal bowl set over simmering water, stirring until smooth. Stir in butter and corn syrup. Cool slightly, then spread just over top of cooled cake.
Cooks' note: Cake improves in flavor if made 1 day ahead and keeps (unglazed or glazed) at room temperature 4 days.

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Nutrition Facts

Per Serving Per 100 g

Amount Per 1 Serving
Calories	532.37 Kcal (2229 kJ)
Calories from fat	240.96 Kcal
		% Daily Value*
Total Fat	26.77g	41%
Cholesterol	126.35mg	42%
Sodium	141.75mg	6%
Potassium	225.65mg	5%
Total Carbs	71.97g	24%
Sugars	42.95g	172%
Dietary Fiber	2.86g	11%
Protein	7.39g	15%
Vitamin A	0.2mg	8%
Iron	1.7mg	9%
Calcium	104.5mg	10%

Amount Per 100 g
Calories	310.63 Kcal (1301 kJ)
Calories from fat	140.59 Kcal
		% Daily Value*
Total Fat	15.62g	41%
Cholesterol	73.72mg	42%
Sodium	82.71mg	6%
Potassium	131.66mg	5%
Total Carbs	41.99g	24%
Sugars	25.06g	172%
Dietary Fiber	1.67g	11%
Protein	4.31g	15%
Vitamin A	0.1mg	8%
Iron	1mg	9%
Calcium	61mg	10%

* Percent Daily Values are based on a 2000 calorie diet. Your daily values may be higher or lower depending on your calorie needs.
Find out how many calories should you eat.