Combine the heavy cream, sour cream and 1/4 cup maple syrup in a large bowl and beat with an electric mixer on medium-high until stiff peaks form, about 4 minutes. Gently fold in the remaining 1/4 cup maple syrup by hand.
Lay 6 cookies out on a large, flat platter in 2 rows of 3, making a rough 7 1/2-by-8-inch rectangle (they should touch but not overlap). Spread a quarter of the maple cream on top. Repeat to make 3 more layers of cookies and maple cream.
Cover the cake loosely with plastic wrap and refrigerate for 6 hours or up to 12 hours to soften the cookies. When ready to serve, sprinkle the top of the cake with the walnuts. Cut into slices with a serrated knife and drizzle each portion with additional maple syrup.
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