Alone, this cake is moist, tender, and elegant, the chocolate echoed by the glossy glaze. But let's put our cards on the table: The exuberant confetti of colorful sprinkles is what makes you want to dive right in. They're just so much fun. Indeed, this cake turns any meal into an occasion with a capital "o."
Sift together flour, cocoa powder, baking soda, baking powder, and salt into a bowl.
03Beat 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).
04Transfer 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.
05Bring 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).
06Carefully peel off parchment from cake. Pour glaze onto center of cake and spread to edges with a spatula. Decorate with sprinkles and/or nonpareils.