Preheat the oven to 350 degrees F (175 degrees C) and position a rack in the center. Lightly butter top surface of a 12-cup muffin pan and put a cupcake liner in each cup.

Whisk together cocoa powder and boiling water in a small heat-safe bowl until smooth. In a medium bowl, whisk together flour, baking powder, and salt.

In the bowl of a stand mixer fitted with a paddle attachment, beat 1/2 cup butter and 1 1/4 cups sugar at medium speed until light and fluffy, 4 to 5 minutes; scrape bowl well. Mix in 2 eggs, one at a time, at medium speed, scraping bowl well after each addition. Mix in 1 1/2 teaspoons vanilla.

Mix in about 1/3 of flour mixture at low speed. Mix in half of cocoa mixture. Repeat, alternating between flour mixture and cocoa mixture, until fully incorporated and batter is smooth. Mix in cream.

Divide batter among prepared muffin cups, filling each about 3/4 full. Bake until a toothpick inserted into center of a cake comes out clean or with a few moist crumbs, 22 to 25 minutes. Cool in pan 5 minutes, then use a small offset spatula or a knife to remove each cake; transfer to a wire rack and let cool completely.

Heat remaining 2/3 cup sugar, 1/3 cup corn syrup, and 1/4 cup water in a small saucepan over medium heat, stirring until sugar is dissolved. When bubbles start to form around edges, stop stirring. Use a pastry brush dipped in water to dissolve any sugar left on sides of pot. Cook, without stirring, until a candy thermometer registers 230 degrees F (110 degrees C).

Meanwhile, put egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment (bowl should be clean and dry). When sugar mixture reaches 230 degrees F (110 degrees C), start beating egg white mixture at medium-high speed.

When sugar mixture reaches 240 degrees F (116 degrees C), remove from heat and add to egg white mixture in a slow, steady stream, beating until soft peaks form, 6 to 8 minutes more. Mix in remaining 2 teaspoons vanilla. Transfer vanilla cream to a disposable pastry bag and cut a 1/2-inch opening from tip.

Use the base of a large round piping tip, a small round cookie cutter, or a paring knife to cut a hole out of the top of each cake. Pipe vanilla cream into holes. Top with a small piece of removed cake to help seal cream inside. Freeze cakes while you make the glaze (at least 15 minutes).

Melt butter cubes, chocolate, and remaining 1 1/2 teaspoons corn syrup in a double boiler or a heat-safe bowl set over simmering water, stirring occasionally, until mixture is smooth. Remove from heat; let cool 5 minutes. Stir before using.