For the cake: Preheat the oven to 350 degrees F. Butter a 9-inch springform pan and line the bottom with parchment. Butter the parchment as well.
In a small saucepan, heat the butter and stout over medium heat until the butter is melted. Set aside to cool slightly.
Meanwhile, in a medium bowl, whisk together the sugar, yogurt, cocoa powder, vanilla, salt and eggs until smooth. Pour in the stout mixture and whisk again until fully incorporated. Add the almond flour, oat flour and baking soda and fold with a spatula to combine. Pour the mixture into the prepared pan and bake until a toothpick inserted in the center comes out with just a few crumbs, about 1 hour. Allow to cool completely.
For the whipped cream: In a medium bowl, use a handheld mixer on medium speed to beat the heavy cream, sugar and whiskey to soft peaks, about 1 minute. Serve the cake dolloped with the whiskey whipped cream.