Preheat oven to 325. Grease the sides of two 8" cake pans, coat with flour, and line with parchment paper, cut to size.
In a medium mixing bowl, sift the 3 cups of flour, 1 tsp baking powder, 1/4 tsp baking soda, and 1/2 tsp salt. Stir until fully combined.
In the bowl of a stand mixer fitted with a paddle attachment, combine 3 cups of granulated sugar with 1 cup of softened butter. Mix on high speed until light and fluffy, 2-3 minutes.
Add one egg at a time (6 eggs total), mixing until fully incorporated between each addition. Add 1 1/2 tsp vanilla and 2 Tbsp orange zest, and mix until combined.
Add 1/2 of the flour mixture and 1/2 cup sour cream, and mix until fully combined. Scrape down the sides, then add the remaining flour and the remaining 1/2 cup sour cream. Mix until fully combined.
Divide the batter between the two prepared 8" cake pans. Bake for 40-42 minutes or until a toothpick comes out clean. Allow to cool. Once cooled, level to 3/4" inch thickness. Then, use your trifle dish as a stencil, and cut the cake to fit inside the trifle dish, trimming off the excess cake as needed. Cakes can be frozen for up to two months.