1. Make the praline: Butter a baking sheet, or line it with a Silpat (a French baking mat). Set aside. In a small, heavy saucepan, bring 1 cup sugar and the water to a boil over medium heat, stirring until sugar dissolves; wash down sides of pan with a pastry brush dipped in water to prevent crystals from forming.

2. Continue cooking, without stirring, until dark amber, swirling pan to color evenly. Remove from heat; stir in nuts, distributing evenly. Immediately pour mixture onto prepared baking sheet. Let sit until hardened and completely cool.

3. Break praline into pieces, and place in the bowl of a food processor fitted with the metal blade. Process to a fine powder. Measure out 1 1/2 cups praline powder, and set aside. Reserve remaining powder for another use.

4. Make the buttercream: In the heatproof bowl of an electric mixer set over a pan of simmering water, whisk together egg whites and remaining 1/2 cup sugar until mixture is warm to the touch and sugar has dissolved.

5. Attach bowl to mixer fitted with the whisk attachment, and beat on high speed until mixture is stiff and cool, about 5 minutes. Add butter all at once, and beat until mixture has a smooth, spreadable consistency, about 5 minutes. Don't worry if the buttercream separates and appears curdled; it will come back together with continued beating.

6. Add praline powder; beat to combine. If using the same day, set aside, covered with plastic wrap, at room temperature, until ready to use. Or refrigerate in an airtight container up to 3 days. Before using, bring to room temperature; beat with paddle attachment until smooth.