4 piping bags; large star tip; 1/2-inch round tips; 2 small French tips
Bring a few inches of water to a boil in a small saucepan that can hold a heatproof bowl without touching the water. Whisk together the sugar, lemon juice, salt and egg whites in the heatproof bowl. Set the bowl above the boiling water and whisk continuously until the mixture is warm to the touch and the sugar is completely dissolved. Transfer to a stand mixer fitted with a whisk attachment and beat at medium-high speed until cool and the whites hold stiff peaks, 10 to 15 minutes.
Toss in a couple of chunks of butter at a time, making sure the pieces are incorporated before adding more. After all the butter is added, continue whisking on medium-high speed; the mixture will deflate and appear curdled. Continue whisking until the buttercream comes back together to form a smooth and spreadable consistency. Add the vanilla and whisk to combine.
Color 3/4 cup of the buttercream with 2 drops of ivory or flesh tone food coloring to create a "skin tone" color. Transfer to a piping bag fitted with a 1/2-inch round tip. Color 1 1/2 cups of the buttercream with 30 drops of red gel food coloring. Transfer to a pastry bag fitted with a small French tip. Fill a piping bag fitted with a large star tip with 1 1/2 cups of the white buttercream. Transfer the remaining white buttercream into a pastry bag fitted with a small French tip.
To assemble Santa's head, arrange the cupcakes on a serving board as follows to make a diamond: one row of 2, one row of 3, one row of 4, two rows of 5, one row of 4, one row of 3 and the last row of 2.
Pipe about 1/2 cup of the ivory buttercream onto the 3 middle cupcakes in row 5 and the 2 middle cupcakes in row 4 below it to create the face of Santa. Spread evenly using a small offset spatula.
Pipe about 1 1/4 cups of the red buttercream in small rosettes covering the 8 cupcakes on the top half of Santa’s head, leaving 1 cupcake unfrosted on the left side (for the white plush of Santa's hat). Leave 2 tablespoons the red buttercream for Santa's mouth.
Pipe 5 large rosettes with the white buttercream with the large star tip to cover the row of 5 cupcakes under the red hat to form the brim. Pipe 1 large white rosette on the unfrosted cupcake to create the plush of Santa's hat.
Pipe small rosettes with the white buttercream with the small French tip to create Santa's beard on the remaining cupcakes, piping closely to avoid any holes. Pipe a row of small white rosettes below the large rosettes to create Santa's hair. Make two curved lines from the middle of Santa's face to his beard to form his mustache and another two small curved lines above Santa's eyes to form his eyebrows.
Place 2 chocolate buttons side by side for Santa's eyes.
Pipe a dot with the ivory buttercream between and below the eyes for his nose. To finish, pipe 2 small curved lines between his mustache and beard with the red buttercream to create his mouth.