Add the butterscotch beer or cream soda to a large pot. Simmer over medium heat until it reduces to 1/2 cup (120 ml). This usually takes around 20-30 minutes. Remove from the heat and let it cool until it's just slightly warm to the touch before using in the cake. (It could take longer than 30 minutes depending on your stove and which soda you are using.)
3 cups (710 ml) butterscotch beer (cream soda will work also)
Preheat the oven to 350°F. Grease a 9x9 inch metal baking pan and line it with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
1 3/4 cups (219 g) all purpose flour, spooned and leveled , 1 1/2 tsp baking powder, 1/4 tsp baking soda, 1/2 tsp salt
In a large bowl cream the butter, brown sugar and granulated white sugar together with an electric mixer, or a stand mixer, on high speed for 1-2 minutes or until fluffy.
1/2 cup (112 g) unsalted butter, softened, 1/2 cup (110 g) light brown sugar, packed, 1/4 cup (50 g) granulated white sugar
Add in the eggs, vanilla and butter extract and mix on medium speed until pale in color, about 1-2 minutes.
2 eggs, at room temperature, 2 tsp vanilla bean paste or extract, 1 tsp butter extract
Add in the butterscotch beer reduction and melted and slightly cooled butterscotch chips. Mix on medium speed until combined.
1/3 cup (66 g) butterscotch chips, melted and slightly cooled
Add in the buttermilk and dry ingredients a little bit at a time, mixing on low speed until all is combined and the batter is smooth. Scrape the sides of the bowl with a rubber spatula as necessary.
1/2 cup (120 ml) buttermilk, at room temperature
Pour the cake batter into the prepared pan. Bake the cake for 27-31 minutes, or until a toothpick comes out clean from the center. Then, let the cake cool in the pan on a wire rack until completely cool.