Black Cat Cookies

In a small saucepan over low heat, heat 3 tablespoons butter and cocoa powder until butter is melted, stirring until combined. Let cool 15 minutes. In a stand mixer bowl fitted with a paddle attachment, beat together remaining butter, espresso powder, sugar, salt, and cooled cocoa powder mixture until well combined and fluffy, about 1 minute. Add yolks and vanilla extract and beat until combined. Add flour in three batches, making sure each addition is fully combined before adding the next. Beat until mixture pulls towards paddle attachment to form a loose ball. Turn dough onto a work surface and form two equal discs. Transfer to refrigerator to chill until dough is firm, about 1 hour. Line baking sheet with parchment paper and heat oven to 375 degrees F. Roll out one disc between parchment paper until just over 1/8" thickness. Peel off top parchment sheet and stamp out cat shapes. Transfer to prepared baking sheet, spaced 1" apart. Bake cookies until center of cookie gives just slightly when pressed, 10 to 12 minutes. Remove from oven and cool on baking sheet 5 minutes before transferring to a wire rack to cool completely. Decorate with white chocolate.