Preheat the oven to 350°F. Grease a 9-inch springform pan or line it with parchment paper.
Scrape the seeds from the vanilla bean. In a large bowl, mix the Greek yogurt, agave syrup, milk, eggs, espresso powder, and vanilla bean seeds until smooth.
In a small pan, melt the butter over low heat. Let it brown slowly until it foams and gives off a slightly nutty aroma. Set aside to cool.
In a separate bowl, whisk together the almond flour, baking powder, baking soda, and salt. Gradually sift the dry ingredients into the wet ingredients. Stir until a uniform batter forms. Gently stir in the browned butter.
Pour the batter into the prepared springform pan and smooth the surface. Bake for 35–40 minutes, or until golden brown and a toothpick inserted into the center comes out clean.
Let the cake cool briefly in the pan, before moving it onto a wire rack to cool completely.
As the cake cools, you can make the topping. In a bowl, cream the mascarpone cheese and powdered sugar together until creamy.
Slowly stir in the cooled espresso until the cream is smooth. Refrigerate for about 10 minutes to firm up slightly.
Spread the mascarpone cream evenly over the completely cooled cake. Optionally, top with chopped dark chocolate before serving.
