In a small saucepan, melt butter over low heat until it starts to sizzle.

In the container of a blender, place egg yolks; process until slightly thickened, 10 to 15 seconds. With processor running, gradually pour lemon juice through food chute, and process until smooth.

With processor running, slowly pour hot butter through food chute. (Mixture will immediately be creamy and thickened.) Add tarragon, and process 5 seconds more. Use immediately.