To prepare cookies: In a medium bowl, whisk together flour, salt and baking soda.
In a bowl of an electric mixer on medium-high speed, beat 1/2 cup butter, 1/2 cup granulated sugar and powdered sugar until light and fluffy, about 2 minutes. Add canola oil, egg, vanilla extract, zest from 2 oranges and 1 teaspoon cardamom, and beat until thoroughly combined. Reduce speed to low, add flour mixture and mix until just incorporated. Cover bowl with plastic wrap and refrigerate for at least 1 hour.
When ready to bake, preheat oven to 350 degrees and line baking sheets with parchment paper.
Shape dough into 1-inch balls. Place dough 2 inches apart on prepared baking sheets.
To decorate cookies: In a small bowl, stir together 1 1/2 tablespoons granulated sugar and 1/2 teaspoon cardamom. Grease bottom of a flat-bottomed glass with butter and dip in sugar-cardamom mixture, then press down cookies. Repeat with remaining cookies. Bake until bottoms are golden brown, about 10 to 12 minutes. Remove from oven, cool 2 minutes and transfer cookies to a wire rack to cool completely. Sprinkle with orange zest, if desired.