Preheat your oven to 325 degrees F. Butter a 9 x 12 inch (or slightly smaller) rectangular pan. Even if your pan is reliably nonstick, butter it, and then set the pan aside.

In the bowl of a stand mixer fitted with the paddle attachment, place the flour, xanthan gum, baking powder, baking soda, salt and granulated sugar and whisk to combine well. Add 1 stick (8 tablespoons; 112g) unsalted butter, milk, egg yolks, vanilla extract and almond extract to the dry ingredients, blending well after each addition. Transfer the batter to a separate large bowl and wash out the bowl of your mixer well, making sure no fat remains in the bowl. Beat the 4 egg whites in the stand mixer with the whisk attachment on high speed until stiff but not dry peaks form (see photo). Scoop half of the egg whites into the bowl of batter, and gently fold in the egg whites until just a few streaks of white remain. Add the rest of the egg whites, and fold again until no white streaks remain. Pour the batter into the prepared pan and shake it gently to create an layer of batter.

Place the pan in the center of the preheated oven and bake, rotating the pan once during baking, for about 30 minutes or until the cake is nicely puffed and pretty uniformly golden brown (see photo) and the cake has begun to pull away from the sides of the pan. Allow the cake to cool for 5 minutes in the pan before transferring to a wire rack to cool completely.

To the bowl of the stand mixer fitted with the paddle attachment, add the remaining 1 stick (8 tablespoons; 112g) unsalted butter and the confectioner's sugar, and mix on high speed until well combined. Lower the mixer speed to low, and drizzle in 4 tablespoons of the butterscotch sauce. Add more sauce by the tablespoon until the mixture is very light brown in color (not yellow – more like a khaki color) and smells and tastes of butterscotch. Take care not to add too much butterscotch in search of a very butterscotch-flavored icing or you will thin the icing too much. You want it to be thickly pourable icing — not fluffy frosting and not a thin liquid. This part is key to the authenticity of your krimpets. Refrigerate the rest of the butterscotch sauce in an airtight container.

With the cooled cake still on the rack, place a piece of parchment paper underneath and pour most of the icing in the center. With a small offset spatula, slowly and carefully spread the icing thickly over the top of the cake. Refrigerate until the icing has set and is nearly dry to the touch.

With a very sharp chef's knife, slice the cake into 9 rectangular pieces. To achieve the traditional zigzag pattern, on each long side of the rectangles, gently use a small cookie cutter to cut two half moons.

Serve just slightly more chilly than room temperature.