I have, though I had to sub pecan for walnut. They were good, but a tad on the dry side.
I like this recipe for brownies better.
1 cup nut butter
1 cup canned pumpkin
1/3 cup honey
1 tsp baking soda
Combine all ingredients in a bowl. Mix well. Spoon into cupcake papers. Bake at 350 degrees for 25 minutes or until a toothpick inserted comes out clean. Makes 8.
Truthfully, I have a really hard time not just eating the batter- lol.