Thumbprint Cookies with White Chocolate-Espresso Ganache

过筛面粉、玉米淀粉、地中海和盐ium bowl; set aside. Beat butter in the bowl of a stand mixer fitted with the paddle attachment on medium-low speed until smooth, about 1 minute. Gradually beat in powdered sugar. Increase mixer speed to medium-high; beat until smooth and light in texture, about 2 minutes. Beat in vanilla. With mixer running on low speed, gradually add flour mixture, beating until dough just starts to clump together, about 1 minute, stopping to scrape sides of bowl as needed. Add walnuts, and beat on low speed until incorporated, about 10 seconds. Cover bowl, and chill at least 4 hours or up to 3 days.

Place white chocolate in a medium-size heatproof bowl; set aside. Heat 3/4 cup heavy cream and espresso in a small saucepan over medium-low, stirring occasionally, until mixture bubbles around edge of pan, 4 to 8 minutes. Remove from heat; let espresso steep 10 minutes. Pour mixture through a fine wire-mesh strainer into a bowl; discard solids. Wipe saucepan clean; return strained cream to saucepan. Add corn syrup and salt. Heat over medium-low, stirring occasionally, until bubbles appear around edge of pan, 4 to 8 minutes. Pour mixture over white chocolate in bowl; stir gently until melted and smooth. Gradually stir in remaining 1/4 cup heavy cream. Cover and chill at least 4 hours or (preferably) up to 12 hours.

Preheat oven to 325°F with oven racks in top and bottom positions. Shape scant tablespoonfuls (about 1/2 ounce each) of cookie dough into balls, and arrange 1 inch apart on 2 ungreased rimmed baking sheets. Using a moistened thumb, press a deep indentation into center of each dough ball. Bake in preheated oven until very light golden around edges, 16 to 18 minutes, rotating pans from top to bottom and turning pans 180 degrees halfway through cook time. Let cookies cool on baking sheets 2 minutes. Transfer cookies to a wire rack, and let cool completely, about 30 minutes.

Transfer white chocolate–espresso ganache to bowl of a stand mixer fitted with the whisk attachment. Beat on medium speed until stiff peaks form, 30 to 45 seconds, being careful not to overbeat. Transfer half of ganache to a piping bag with a 1/2-inch hole cut in the corner. Pipe about 2 teaspoons white chocolate– espresso ganache into indentation of each cookie. Repeat with remaining ganache.