For the whiskey caramel sauce: Combine the sugar and 1/4 cup water in a medium saucepan over medium-high heat, swirling the pan but not stirring, until the mixture is dark amber, 7 to 10 minutes. Turn off the heat and whisk in the heavy cream, vanilla and salt until smooth. Whisk in the whiskey. Transfer to a bowl and cool completely (the caramel will thicken as it cools).
For the chocolate stout cakes: Position a rack in the center of the oven and preheat to 350 degrees F.
Melt the butter in a medium saucepan over medium heat until it just begins to bubble, about 5 minutes. Add the cocoa powder and whisk until dissolved and the mixture is smooth, about 30 seconds. Remove from the heat and whisk in the stout beer. Set aside to cool slightly, about 5 minutes.
Meanwhile, spray a 12-cup muffin tin generously with nonstick canola oil spray and wipe out any oil that pools at the bottom of the cups. Dust the muffin cups with cocoa powder and tap out the excess.
Whisk the flour, brown sugar, baking soda and salt in a medium bowl.
Whisk the egg, Greek yogurt and vanilla extract in large bowl.
Whisk half of the stout beer mixture into the egg mixture and blend until combined. Whisk in the remaining half of the stout beer mixture and blend until smooth. Add the flour mixture and blend until just combined.
Divide the batter evenly into the prepared muffin tin. Bake the cakes, rotating the muffin tin halfway through, until a tester or toothpick inserted into the center comes out clean, 20 to 25 minutes. Transfer the muffin tin to a wire rack and let the cakes cool in the muffin tin for 10 minutes.
Loosen the edges of each cake with an offset spatula, then carefully remove the cakes from the muffin tin. Serve the warm cakes upside-down on individual plates drizzled with the Whiskey Caramel Sauce and sprinkled with the toffee bits.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)