Step 1

Preheat the oven to 325 degrees F and grease an 8- or 9-inch square metal or glass baking pan.

Step 2

With an electric hand mixer, beat the egg yolks and sugar together in a large bowl on low speed for about 1 minute, until the yolks and sugar form a pale, thick paste.

Step 3

Beat egg mixture for magic cake

Step 4

Slowly pour in the melted butter and the vanilla extract and mix until incorporated.

Step 5

Whisk the flour and salt together in a small bowl.

Step 6

By hand, whisk 1/3 of the flour into the batter until blended, then whisk in 1/3 of the milk. Repeat with the flour and milk until all ingredients are blende

Step 7

With an electric hand mixer or stand mixer fitted with the whisk attachment, beat the egg whites for about 1 minute on medium-high speed until thick and foamy.

Step 8

Add the cream of tartar and beat for about 1 minute more until stiff peaks form. (If you don't have a whisk attachment for your hand mixer, clean the beaters and dry very well before using them to beat the eggs; it may take more time to achieve stiff peaks without a whisk.)

Step 9

Gently and slowly fold the egg whites into the liquid batter with a spatula. An easy technique for folding is to scrape around the edge of the bowl with the spatula, then "fold" the batter into the middle of the bowl on top of itself.

Step 10

Gently fold egg whites

Step 11

Pour the batter into the greased pan and bake for about 45 minutes to 1 hour. Timing will vary based on your oven and the pan used. The cake is done when it has risen high in the pan with a golden brown top crust, and barely jiggles when you gently shake the pan.

Step 12

Transfer to a wire rack and cool completely in the pan.

Step 13

When the cake is cool, make the ganache: Pour the chopped chocolate into a medium bowl. Heat the cream to a simmer, then pour over the chocolate. Let the chocolate and cream rest for 1-2 minutes, then gently stir until the chocolate is completely melted.

Step 14

Pour the ganache over the cake and gently spread with a spatula until the cake is evenly covered.

Step 15

Sprinkle with rainbow sprinkles or other decoration as desired.

Step 16

Refrigerate for at least 2 hours until the ganache has hardened.

Step 17

Slice and serve.