Beat the softened butter and sugar in a large mixing bowl until the mixture looks lighter and fluffy. This creates the airy foundation for the cupcake batter.

Blend the vanilla into the butter mixture. The flavoring should disperse evenly without leaving wet streaks.

Beat in the eggs individually, fully incorporating each before adding the next. The batter should look blended after each addition rather than streaked with egg.

Whisk the flour, baking powder, and salt together in another bowl. Evenly distributed leavening helps the cupcakes rise consistently.

Incorporate the dry mixture in three additions, alternating with buttermilk and keeping dry ingredients first and last. Mix briefly after each addition, stopping when no dry patches remain.

Lightly grease the muffin pan, add cupcake liners, and portion the batter until each liner is 3/4 full. Leave the remaining space open so the batter has room to rise.

Bake at 350°F for 18–25 minutes, checking for a clean toothpick before removing the cupcakes and letting them cool completely. Look for lightly golden edges and a center without wet batter.

Whisk the egg yolks thoroughly in a medium bowl. They should become uniform, with no separate streaks of yolk.

Whisk the milk into the yolks gradually. The mixture should look evenly blended before it reaches the saucepan.

Combine the sugar, cornstarch, and salt in a large, heavy saucepan over medium-low heat. Break up visible starch clumps so they don’t remain dry when liquid is added.

Stir the milk-and-yolk mixture into the saucepan in a gradual stream. Keep the mixture moving so the cornstarch disperses without forming lumps.

Stir constantly over medium-low heat for about 20 minutes, removing the pan from the burner when the filling thickens and reaches a gentle boil. Large, slow bubbles should break through the thickened surface, and the filling will firm further as it cools.

Set the saucepan in a large bowl of ice water, continue stirring, and blend in the vanilla. The filling should cool evenly instead of staying hot around the edges of the pan.

Press plastic wrap against the filling’s surface and refrigerate until needed for assembly. Direct contact keeps a tough skin from forming as the pastry cream chills.

Pulse the chocolate in a food processor until finely chopped, then place it in a mixing bowl. Small, evenly sized pieces melt more readily when the cream reaches them.

Warm the cream in a small saucepan, stirring frequently, and remove it from the heat when steam appears. The cream should be hot enough to melt chocolate without reaching a vigorous boil.

Whisk the hot cream gradually into the chocolate, then let the smooth ganache cool until comfortable to handle but still fluid. It should flow in a thick ribbon rather than splash like hot cream or hold a stiff mound.

Load the chilled pastry cream into a piping bag fitted with a filling tip once the cupcakes have cooled completely. The filling should hold its body, and the cake should feel cool throughout.

Insert the tip into each cupcake and gently pipe in pastry cream, stopping before the cake splits. A little resistance or slight swelling signals that the center is filling.

Dip the cupcake tops into the fluid ganache and leave the coating to set for 30 minutes to an hour. The finished topping should settle into a smooth layer without streaming down the sides.