01Preheat oven to 350 F. Using a stand mixture on medium speed, cream together butter and both sugars until smooth.
02Add vanilla, yogurt or sour cream, and eggs, and mix until thoroughly combined.
03In a separate, medium-size bowl, stir together flour, baking soda and salt until combined.
04Add half the dry ingredients to the butter mixture and mix. When mostly combined, add the remaining dry ingredients. Stir mixture until soft, sticky dough forms and no dry ingredients are visible.
05Using a spoon or a small cookie scoop (about 1 inch across), place dough onto greased or parchment-paper-lined baking sheets, 2 inches apart. Bake for approximately 11-13 minutes until golden brown. Place cookies on rack and let cool to room temperature.
06For the topping, place chocolate chips and milk in a small microwavable bowl and microwave on high setting for 30 seconds. Stir, and if chips aren’t melted, return to microwave for another 10 seconds until melted. Stir until chocolate is smooth. Use the spoon to drizzle chocolate in stripes over the cookies. If chocolate starts to harden, re-melt in microwave.
07After chocolate on the cookies hardens, use a sifter or a small mesh sieve to dust the cookies with powdered sugar.
08Store in airtight container. To store longer than two days, refrigerate or freeze the cookies.