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1
Preheat oven to 350u00b0F with rack in middle. Butter bottom and side of pan, then line bottom with a round of parchment.
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2
Sift together flour, cocoa powder, baking soda, baking powder, and salt into a bowl.
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3
Beat together butter and brown sugar in a large bowl with an electric mixer at medium-high speed until light and fluffy, 3 to 5 minutes. Add eggs 1 at a time, beating well after each addition, then beat in vanilla. Reduce speed to medium-low and add flour mixture and milk alternately in batches, beginning and ending with flour mixture (batter will appear curdled).
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4
Transfer batter to cake pan and smooth top. Bake until cake begins to pull away from side of pan and a wooden pick inserted into center comes out clean, 35 to 40 minutes. Cool cake in pan 5 minutes, then invert onto a rack and cool completely, about 1 hour.
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5
Bring cream to a simmer in a small heavy saucepan over medium heat, then pour over chocolate in a bowl and let stand 1 minute. Gently whisk until smooth, then stir in corn syrup. Cool completely, gently stirring occasionally, about 30 minutes (glaze will thicken).
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6
Carefully peel off parchment from cake. Pour glaze onto center of cake and spread to edges with a spatula. Decorate with sprinkles and/or nonpareils.
