In bowl of electric mixer fitted with paddle attachment, beat sugar and butter 2 minutes at medium speed or until well combined, scraping down sides of bowl occasionally. Add vanilla; beat until well combined. Scrape down sides and bottom of bowl.

Add flour mixture and beat on medium speed 1 to 2 minutes or until mixture comes together and forms a thick cookie dough.

Form batter into tablespoon-size balls (13g if you are scaling), and place on prepared pan. You should have 15 balls. Lightly flatten with your fingers. Transfer to oven and bake 9 to 11 minutes or until bottoms of cookies are golden brown. Transfer pan to wire cooling rack and cool cookies on pan completely before making rum balls. Cookies can also be stored in zip-top bag at room temperature up to 2 days before making rum balls.

With mixer running on low speed, gradually add rum, a tablespoon at a time, until mixture starts to come together. Start with 2 tablespoons, and add rum in 1 teaspoon increments after that, if needed. Mixture should be wet enough that it can be shaped into balls, but not wetter than that. Transfer to refrigerate and chill at least 30 minutes before rolling.

Line a plate or small baking pan with parchment paper. Use your hands to shape mixture into 1-inch balls (you should have about 15). Roll each ball in sanding sugar or powdered sugar.

Place rum balls on prepared plate and transfer to refrigerator 2 hours to firm up. Then, transfer rum balls to covered container and refrigerate  up to 1 week. Serve chilled or at room temperature.