Chocolate Cake with Ganache and Praline Topping

For cake:

Preheat oven to 350°F. Spray 9-inch cake pan with nonstick spray. Line with parchment. Spray parchment. Dust with flour, tapping out any excess.

Place butter and cocoa powder in medium bowl. Pour 1/2 cup boiling water over; stir. Let stand 2 minutes; whisk until blended. Whisk flour, sugar, baking soda, and coarse salt in another medium bowl. Whisk buttermilk, egg, and vanilla in large bowl. Gradually whisk cocoa mixture into buttermilk mixture; whisk until smooth. Add flour mixture in 3 additions, whisking to blend between additions (batter will be thin). Pour batter into prepared pan.

Bake cake until tester inserted into center comes out clean, about 30 minutes. Cool in pan 10 minutes. Run knife around pan edges to release cake. Invert onto rack; remove pan and parchment. Cool completely.

For ganache:

Place chocolate and cream in microwave-safe bowl. Heat in microwave in 15-second intervals, stirring occasionally, until melted and smooth. Stir in butter. Let stand until spreadable, about 30 minutes. Spread over top and sides of cake. Transfer cake to plate. Chill 2 hours and up to 1 day.

For praline:

Stir first 3 ingredients in heavy large saucepan over medium-high heat until butter melts. Boil 1 minute without stirring. Remove from heat; whisk in sugar and vanilla. Add pecans; stir just to incorporate. Quickly pour praline over cake. Spread just to edges (topping sets quickly). DO AHEAD: Can be made 1 day ahead. Cover with cake dome. Store at room temperature.