Preheat the oven to 350 degrees F, and line baking sheets with parchment paper.

Place the flour, sugar, cocoa powders, baking powder, baking soda, and salt in a large mixing bowl, and whisk to combine.

Add the soft butter, and mix on medium speed using the paddle attachment for about a minute.  The mixture should resemble damp sand.

Stir in the eggs until incorporated.

Scrape the bottom and sides of the bowl with a silicone spatula, and add the Greek yogurt and vanilla.

Mix on medium-high speed for about a minute and a half, to aerate the batter and build the cake’s structure.

Using a 1-ounce scoop, drop balls of batter on the prepared sheet pans.

Bake for 10 to 12 minutes, or until the cakes spring back when lightly pressed.

Cool completely, then sandwich around 1 ounce of filling.

li>Place half the sugar in a small pot with the water, and cook over medium heat.

Place the egg whites, cream of tartar, and salt in a large mixing bowl and whip on medium-high speed with the whisk attachment.

When the whites can hold soft peaks, slowly add the remaining sugar (about a tablespoon at a time) while continuing to whip.

When all the sugar has been added, add the vanilla and continue to whip the meringue until stiff.  Turn the mixer down to low speed while you wait for the sugar syrup to come to temperature.

When the sugar syrup has reached 235 degrees F, remove it from the heat, turn the mixer back up to medium-high, and slowly pour the hot syrup down the side of the mixing bowl in a thin stream.  (Do not pour onto the spinning whisk; it could splash out of the bowl and burn you.)

Continue to whip until there is no hint of warmth when you place your hand on the side of the bowl.

When the meringue is completely cool, whip in the butter, a tablespoon at a time.  (If the mixture seems runny, place the entire bowl and whisk in the refrigerator for 10 to 20 minutes, and re-whip.  If the mixture seems curdled, just keep whipping- it will come together.)