Line an 8½ × 4½-inch loaf pan with cooking spray and acetate. Place the chocolate in a heatproof mixing bowl. In a small saucepan, combine the milk, heavy cream, and espresso, bring to a simmer, and pour over the chocolate. Rest for 30 seconds, then whisk until it is smooth and shiny. Whisk in the butter in 3 additions, until well combined. Pour into the prepared loaf pan, wrap with plastic, and transfer to the freezer until solid, about 3 hours. Loosen the sides of the frozen ganache with an offset spatula and flip it onto a cutting board. Peel away the acetate and cut into at least eight 4 × ½-inch rectangles, transfer to a tray, wrap in plastic, and refrigerate until ready to use.