Place walnuts in a food processor. Process until roughly ground.
Melt 3/4 cup butter in a skillet over medium heat. Stir in 1/2 cup sugar until dissolved, about 1 minute. Remove from heat. Add ground walnuts and maple flavoring; stir to combine.
Mix 2 cups flour, yeast, 3 tablespoons sugar, and salt together in a bowl.
Combine sour cream, 1 cup butter, and water in a saucepan over low heat. Warm until melted, about 2 minutes; do not boil. Remove from heat; let stand until cool enough to dip a finger in, about 3 minutes.
Add sour cream mixture to the flour mixture slowly. Beat with an electric mixer on medium speed for 2 minutes, scraping the bowl as you go. Add eggs and 1 additional cup flour; stir briefly to combine. Beat on high speed for 2 minutes more. Stir in remaining flour 1 cup at a time until a soft dough forms.
Turn dough out onto a floured surface; knead several times and roll into a ball. Let rise for 10 minutes. Grease 2 jelly roll pans.
Divide dough into 4 pieces; roll each out to a 14x12-inch rectangle. Spread 1/4 of the filling over each rectangle, leaving a 1-inch space around all edges. Roll each up lengthwise, jelly-roll fashion; pinch seams closed. Place, seams down, on the prepared pans. Cover with a damp cloth; let rise in a warm place for 1 hour.
Preheat oven to 350 degrees F (175 degrees C).
Bake in the preheated oven until golden, about 35 minutes. Place rolls on a rack to cool completely, 2 to 3 hours.
Do not use English black walnuts; the taste is too strong.
Make the filling first, as it needs to cool. For an alternate filling, combine one 25-ounce can poppy seed filling, 1/4 cup raisins (optional), 2 to 3 tablespoons honey, 1 tablespoon softened butter, 1/2 teaspoon lemon zest, and 1/4 teaspoon ground cloves.
This bread will be lighter when baked on less-humid days and denser when baked on humid or rainy days. I prefer the dense bread, so I now bake the bread with a pan of water at the bottom of the oven.
Per Serving: 302 calories;21.8 g fat;
22.6 g carbohydrates;
5.7 g protein;
42 mg cholesterol;
138 mg sodium.