chocolate pretzel cookies

INGREDIENTS

• 1 teaspoon good-quality instant espresso powder
• 3 tablespoons boiling water
• 1/2 cup (1 stick) unsalted butter, softened
• 1/2 cup granulated sugar
• 1 teaspoon pure vanilla extract
• 1/2 teaspoon coarse salt
• 1 large egg
• 1/2 cup unsweetened Dutch-process cocoa powder
• 1 3/4 cups all-purpose flour
• 1 large egg yolk
• Sanding sugar, for sprinkling