- Nonstick vegetable oil spray
- 1/2 cup (1 stick) unsalted butter, cut into 1/2-inch cubes, room temperature
- 1/4 cup natural unsweetened cocoa powder
- 1/2 cup boiling water
- 1 cup all purpose flour
- 1 cup sugar
- 1/2 teaspoon baking soda
- 1/4 teaspoon coarse kosher salt
- 1/4 cup buttermilk
- 1 large egg
- 1/2 teaspoon vanilla extract
- 1 cup semisweet chocolate chips
- 3 tablespoons heavy whipping cream
- 2 tablespoons (1/4 stick) unsalted butter, cut into 1/2-inch cubes
- 3/4 cup (packed) dark brown sugar
- 1/4 cup heavy whipping cream
- 3 tablespoons unsalted butter
- 3/4 cup powdered sugar
- 1 teaspoon vanilla extract
- 1 cup pecans, toasted , chopped
- 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.
- 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.
- 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.