Heat oven to 350°.
Butter a spring form pan, place an 12-inch circle of parchment paper on the bottom and up the sides, butter the parchment paper, and dust the pan with rice flour.
In a Cuisinart, process the walnuts and chocolate to a medium fine texture.
In a mixer, beat egg yolks and sugar to a thick ribbon stage. Add butter and beat until smooth.
Fold the walnut and chocolate mixture into the emulsion, then beat for 2 to 3 minutes.
Pour mixture into the prepared springform pan and bake for 45 min. After cooling for 5 minutes, remove from the pan.
Cool completely, slice, and serve with a sprinkling of powdered sugar.