Prepare for baking: Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.

Bloom the cocoa for rich flavor and moisture: In a large bowl, whisk together the unsweetened cocoa powder and hot water (or hot coffee) until completely smooth. This crucial step, called blooming, intensifies the chocolate flavor and significantly contributes to a fudgy, moist crumb. Set aside to cool slightly.

Combine dry ingredients: In a separate medium bowl, whisk together the all-purpose flour, granulated sugar, baking soda, and salt.

Combine wet ingredients: To the bloomed cocoa mixture, add the eggs, vegetable oil, buttermilk, and vanilla extract. Whisk until all ingredients are well combined.

Mix for ultimate tenderness and fudginess: Gradually add the dry ingredients to the wet ingredients. Mix on low speed with an electric mixer or by hand until just combined. It is essential to avoid overmixing at all costs; overmixing develops gluten and can result in tough, dry cupcakes. A few small lumps in the batter are perfectly fine and are actually a sign of proper mixing for a tender, moist cupcake.

Bake the cupcakes: Divide the batter evenly among the prepared cupcake liners, filling each about two-thirds full. Bake for 18-22 minutes, or until a wooden skewer inserted into the center of a cupcake comes out with moist crumbs attached, but no wet batter. Never overbake, as this is the biggest factor in dry cupcakes!

Cool completely: Let the cupcakes cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. This helps prevent them from becoming soggy at the bottom.

Make the Mint Buttercream: In a large bowl, beat the softened butter with an electric mixer on medium speed until creamy and pale, about 2-3 minutes. Gradually add the sifted powdered sugar, one cup at a time, beating until smooth after each addition. Beat in the milk or cream, pure mint extract, green food coloring (if using), and salt. Increase the mixer speed to high and beat for another 2-3 minutes until the buttercream is wonderfully light and fluffy.

Decorate: Once cupcakes are completely cool, pipe or spread the mint buttercream generously onto each cupcake. Top each with 2-3 small chocolate squares or chunks. Lightly dust with ground cinnamon or cocoa powder for an extra touch of flavor and visual appeal.