Preheat oven to 350° F. Grease 24 wells in standard-size muffin pans.

Melt butter in saucepan over moderate heat. Add sugar and stir to combine. Continue heating the mixture until it is hot but not bubbling; it will become shiny as you stir it.

Remove from heat and stir in cocoa powder and vanilla extract. Add eggs, one at a time, beating until smooth after each addition. Add salt and baking powder, stirring until combined. Add flour, ¼ cup at a time, stirring until the flour is incorporated.

Divide the batter between the prepared muffin pans, adding about 1 1/2 to 2 tablespoons batter to each well. Scatter chocolate chips over the top of each brownie.

Bake until just set, about 10 minutes. Place pan on rack to cool for about 10 minutes. Remove brownies from the pan with a thin spatula or with skewers.

For the buttercream, place raspberries in a mesh strainer over a bowl. Press the berries against the side with a spatula to release their juices into the bowl.

In the bowl of a standing electric mixer, combine butter, 3 cups of the confectioners' sugar, the raspberry juice, and framboise or Chambord, if using. Mix slowly with the paddle attachment to combine. Add another cup of confectioners' sugar and mix until incorporated. Increase speed to medium-high and beat until smooth. Add additional powdered sugar about ½ cup at a time to achieve a smooth, creamy consistency. Mix in milk or cream to thin frosting if needed.

Pipe frosting on cooled brownies or spread it on generously with a thin, flexible spatula. Garnish with fresh raspsberries, sprinkles, or both.