Start by preheating your oven to 350°F. Grease and flour two 8-inch round cake pans, then line the bottoms with parchment paper circles to ensure easy release.

In a medium bowl, whisk together the flour, baking powder, salt, and cinnamon until well combined.

In a large mixing bowl, beat the softened butter on high speed with an electric mixer until creamy and smooth, about 1 minute. Add both brown and granulated sugars and continue beating on high for 3 to 4 minutes, until the mixture is pale and fluffy. Add in the vanilla extract and eggs, then beat on medium-high until fully incorporated.

Switch the mixer to low speed and add the dry ingredients in three parts, alternating with the sour cream. Start and finish with the flour mixture, mixing just until no streaks of flour remain. The batter will be thick and rich—do not overmix.

Evenly divide the batter between the two prepared pans and smooth the tops. Bake for about 35 minutes, or until the cakes are golden brown and a toothpick inserted in the center comes out clean. Let the cakes cool in the pans for 20 minutes before turning them out onto wire racks to cool completely. For a smoother frosting experience, wrap and freeze the layers until firm.

In the bowl of a stand mixer fitted with the paddle attachment, combine the sifted powdered sugar and softened butter. Mix on low to incorporate, then increase to medium speed and beat for 2 more minutes until fluffy. Add the salt, vanilla extract, and 1/3 cup of the butterscotch sauce. Beat for another 2 minutes until the frosting is smooth and evenly blended.

Use a serrated knife or cake leveler to slice each cooled cake horizontally in half, giving you four thin layers. Place one layer on a cake stand or turntable. Spread about 1/3 cup of butterscotch sauce across the surface, stopping about 1/2 inch from the edges.

Repeat the layering process—cake, butterscotch sauce—until all layers are stacked. Position the final layer upside-down for a flat, smooth top.

Using an offset spatula, apply a thin layer of frosting all over the cake to trap any crumbs. This is your crumb coat. Chill the cake in the refrigerator for 30 minutes or in the freezer for 15 minutes until the layer is firm to the touch.

Finish by applying the remaining frosting in an even, smooth layer around the cake. Decorate as desired, using swirls, drips, or extra butterscotch sauce if preferred.

Serve immediately or store the finished cake in the refrigerator for up to 2 days. Allow it to come to room temperature for about an hour before slicing and serving.