In theory large egg yolks should be the same size and weight, but in reality they are not. Therefore, I would encourage you to purchase a kitchen scale to weigh not just the yolks in this recipe but to weigh all of your ingredients baking ingredients.
Also, I learned a trick from Rose Levy Barenbaum. Add corn syrup to the sugar before heating. The corn syrup allows the sugar to come to a temperature of 240°F without the need for an instant read thermometer. I used one anyway the first time to get a sense of exactly how big the bubbles become when 240°F is reached.
6 lg (4 oz/112 g) egg yolks (egg whites can be frozen for another use)
1/2 teaspoon kosher salt
3/4 cup (5 1/4 oz/150 g) granulated sugar
1/2 cup (6 oz/166g) corn syrup
2 cups (1 lb/450 g) unsalted butter*, softened
1/2 cup (4 oz/115 g) mascarpone*
6 oz bittersweet chocolate, melted and cooled
Place the egg yolks and salt in the bowl of a stand mixer and using the whisk attachment whip them until pale yellow in color. While the yolks are whipping, in a small pan heat sugar and corn syrup; stirring with a heat-proof spatula to dissolve the sugar. Once the sugar is dissolved stop stirring and allow the syrup to come to a full boil (meaning large bubbles are covering the entire surface of the syrup).
After the eggs are whipped stop the machine and pour a small amount of the hot sugar syrup into the eggs. Quickly whip the mixture. Continue stopping the machine, pour in another tablespoon and whip. As the eggs heat up (test by touching the outside of the bowl with your hand) you can add more at a time. I found that by stopping the machine each time less sugar syrup gets spun onto the inside bowl.
When all the sugar syrup has been added, increase speed to medium-high and whip the egg/sugar mixture until the bowl is cool to the touch. Reduce the mixer speed to medium and add a couple of tablespoons of butter at a time incorporating each before adding the next knob of butter. After all the butter has been incorporated, add the chocolate (and mascarpone, if using.) Return the mixture to medium-high and whip until the buttercream is fluffy, a couple of minutes. Frost the cake and serve.
*Variation: Reduce the butter to 1 3/4 cups (12 oz/337.5 g) and increase the mascarpone to 1 cup (8 oz/230 g). Continue the recipe as above.