Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Line a standard muffin tin with paper liners.

In a medium bowl, whisk together cocoa powder and hot water until smooth. Set aside.

In a separate bowl, whisk together flour, baking soda, baking powder, and salt.

In a saucepan, melt butter with sugar over medium-low heat, stirring until combined. Remove from heat and pour into a mixing bowl.

With an electric mixer on medium-low speed, beat the butter mixture until cooled, about 4-5 minutes.

Add eggs, one at a time, beating until each is fully incorporated. Scrape down the sides of the bowl as needed.

Add vanilla extract, then add the cocoa mixture and beat until well combined.

Reduce the mixer speed to low. Add the flour mixture in two batches, alternating with the sour cream, and beating until just combined after each addition.

Divide the batter evenly among the lined muffin cups, filling each about three-quarters full.

Bake for about 20 minutes, rotating the tin halfway through, or until a cake tester inserted into the center comes out clean.

Transfer the muffin tin to a wire rack and let cool for 15 minutes. Then, turn the cupcakes out onto the rack to cool completely.

To make the buttercream frosting, beat the butter with an electric mixer on medium-high speed until pale and creamy, about 2 minutes.

Reduce speed to medium. Gradually add confectioners' sugar, about 1/2 cup at a time, beating well after each addition. Beat for about 5 minutes total.

After every 2 additions of sugar, increase speed to high for 10 seconds, then reduce speed to medium-high.

Add vanilla extract and beat until the buttercream is smooth.

Add food coloring (optional) and mix until evenly colored.

Pipe the frosting onto the cooled cupcakes using a piping bag and desired tip.

Decorate with colored sugar sprinkles (optional).

If not using the frosting immediately, cover and refrigerate for up to 3 days. Bring to room temperature and beat on low speed until smooth before using.