Preheat oven to 350°F. Spray two 12-count mini muffin pans very well with cooking spray (or grease and flour the pans); set aside.

Put 1 stick (½ cup) unsalted butter and 6 ounces chopped dark or bittersweet chocolate in a large microwave-safe bowl.

Microwave on high for 1 minute, stir, then continue in 15-second increments, stirring after each, until the butter and chocolate are melted and the mixture is smooth.

Let the chocolate mixture cool for 2–3 minutes (until warm but not hot) so the eggs won't scramble when added.

Add 2 large eggs, ¾ cup granulated sugar, 1 tablespoon vanilla extract, and if using, 1 tablespoon brewed coffee and 1 teaspoon instant espresso granules. Whisk vigorously until the mixture is evenly combined and slightly glossy.

Add ¾ cup all-purpose flour and ½ teaspoon salt. Stir until just combined and smooth—do not overmix.

Using a spoon or small scoop, fill each mini muffin cavity about 3/4 full with batter.

Place 1 unwrapped Rolo candy into the center of each filled cavity and press it down until it is flush with the batter (it's okay if the cavities are nearly full).

Bake in the preheated oven for 11–14 minutes, beginning to check at 11 minutes. A toothpick inserted into the brownie (avoiding the candy center) should come out clean or with a few moist crumbs. Watch closely, as mini brownies can go from underdone to overdone quickly.

Remove the pans from the oven and set them on a wire rack. Cool in the pans for about 10–15 minutes or until the brownie bites are cool enough to handle and release from the pan.

Remove the brownie bites from the pans and let them cool completely on the wire rack. If desired, top each with whipped topping and sprinkle with chocolate sprinkles before serving.