Preheat oven to 425ºF. Place 9 free-standing muffin cups* on a baking sheet. 

Brown the butter: Place the sliced butter in a small skillet set over medium heat. Cook, stirring occasionally, until the butter melts and starts to foam up. Continue to cook, whisking often, just until golden-brown specks appear. Remove from heat immediately.

Whisk the vanilla extract into the brown butter. Transfer half of the mixture (about 1 1/4 ounces) into a separate bowl. Add the chocolate chips or chopped chocolate to the remaining butter and let sit for a couple minutes to give the chocolate time to melt. Then, whisk until smooth. Set aside.

In a clean, medium bowl, whisk together 1 1/2 cups (6 ounces) almond flour or hazelnut meal, 2/3 cup (4 5/8 ounces) granulated sugar, and salt, making sure there are no lumps in the mixture.

In a second medium bowl, whisk the egg whites until very foamy on the surface (but still liquid). Add this to the dry ingredients and fold in with a spatula just until fully incorporated. Divide the mixture evenly between two separate bowls (about 8 1/4 ounces of batter per bowl).

To one batch of batter, add the remaining 1/4 cup (1 ounce) almond flour/hazelnut meal and the plain brown butter, folding in just until all ingredients are evenly incorporated. This will be the vanilla portion of your marble financiers.

To the second batch of batter, add the cocoa powder, remaining 1 tablespoon granulated sugar, espresso powder, and the chocolate butter mixture. Fold in just until all ingredients are evenly incorporated. This will be the chocolate portion of your marble financiers.

Carefully spoon alternating spoonfuls of chocolate and vanilla batter into the muffin cups, making sure that each one has both chocolate and vanilla batter visible on top when you're done. The batter is rather thick, so take time to make sure it fills the bottoms and sides of the muffin cups as best as possible. Once done, use a toothpick or knife to briefly swirl the batter in each cup. 

Transfer the baking sheet with the muffin cups into the preheated oven. Bake at 425ºF for 10 minutes, and then reduce the heat to 375ºF and bake for another 10-12 minutes, or until a toothpick inserted into the center of one of the financiers comes out clean or with only a couple crumbs. Cool the financiers on the baking sheet briefly, and then move to a wire rack to cool completely. 

Store in an airtight, zip-top bag at room temperature.