Step 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.

Step 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.

Step 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.

Step 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.

Step 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.

Step 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.