To make the cake:.
Preheat oven to 350. Spray a 9 inch cake pan with non-stick spray. Line with parchment paper. 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 another 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 in 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.
To make the 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. Chill 2 hours or up to 1 day.
To make the 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 mixture over cake. Spread just to edges (topping sets very quickly).
Can be made a day ahead. Cover with cake dome and store at room temperature.