Preheat the oven to 325°F (165°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the long sides so the cake can be lifted out easily.
In a large bowl, whisk together the almond flour, coconut flour, granulated sweetener, baking powder, xanthan gum and salt. Add the melted butter, eggs, sour cream, almond milk and vanilla extract and mix until smooth.
Spread the batter evenly in the pan and bake for 20-24 minutes, until golden and set in the center. Cool completely in the pan.Lift the cake out using the parchment and cut it in half crosswise to make two equal rectangles.
For the vanilla cream, add the cold mascarpone, cold heavy cream, powdered sweetener and vanilla extract to a large bowl. Beat on low for 30 seconds, then on medium-high for 2-3 minutes until thick, stiff peaks form.
Place one cake rectangle on a serving platter and spread all of the vanilla cream over it in a thick, even layer. Gently set the second rectangle on top and refrigerate for 30 minutes.
For the ganache, heat the heavy cream until just simmering and pour it over the chocolate chips and butter. Let it sit for 2 minutes, then stir until smooth and glossy. Cool for about 10 minutes, until thickened but still spreadable.
Spread the ganache over the top of the cake. Refrigerate for at least 4 hours, or overnight, so the layers soften slightly and slice cleanly like a real eclair.