In a large bowl, using an electric mixer, beat the 2 tablespoons unsalted butter with the 1 large egg, then beat in the ¼ cup granulated sugar, ¼ teaspoon ground cinnamon, 3 tablespoons almond flour, and 1 teaspoon baking powder. Now beat in the ground 2 ¼ cups walnut pieces and mix on low until smooth. Place the bowl in fridge and chill for 30 minutes.
When ready to bake, preheat oven to 350° F and line two baking sheets with parchment paper. Place 1 cup powdered sugar in a bowl.
Remove the dough from the fridge and roll into 1-inch balls. Roll in sugar and place on the baking sheets about 1 inch apart. Place in the oven and bake for about 15 minutes. Remove from oven, let cool on the baking sheets for 5 minutes, then cool completely on a wire rack.
Notes
Don't over-process the walnuts. It's a fine line between ground walnuts and walnut butter. Make sure not to overdo it in the food processor. You want the walnuts to retain some texture and not turn into nut butter.
Cool the cookies until firm. Note that the cookies will still be a bit soft when pulled from the oven. Cool them for 5 minutes on the baking sheet, then transfer to a wire rack to cool completely. They will set as they cool.
Make them vegan. Substitute non-dairy butter or coconut oil for the butter and omit the egg. The cookies will be a bit more delicate but still delicious.
Leave out the powdered sugar. I love the cool sweetness the powdered sugar brings to these cookies. However, for a less-sweet cookie, you can skip rolling them in sugar before baking.