Make Yellow Cake*: Preheat the oven to 350°F and prepare a 9x13 pan by either lining it with parchment or coating it with cooking spray.
Combine flour, sugar, baking powder, baking soda, and salt in a small bowl and set aside.
Whisk together the oil, eggs, vanilla, and buttermilk in a medium bowl.
Add the dry ingredients to the wet mixture and whisk until no flour is visible and no lumps are remaining, but do not over-mix.
Pour batter into the prepared pan and spread with a spatula until the batter is smooth on top and evenly distributed. Bake for 15-20 minutes, rotating after 8 minutes. The cake is done when a toothpick shows a few moist crumbs with no batter remaining. The top will be lightly golden.
Let cool on a rack before transferring from pan to a cutting board. Cut cake into 1-inch cubes, cover with a dish towel or plastic wrap, and set aside.
Macerate Strawberries: Toss cut strawberries together with sugar and lemon juice. Cover and store in the fridge until needed.
Make Whipped Cream: Place heavy cream in a medium bowl or stand mixer and whip on medium-high speed with a whisk attachment until it has reached the “soupy peaks” stage - the whisk will leave an impression in the cream and it will have increased in volume but not quite form peaks yet.
Add powdered sugar, vanilla, and salt. Whip on medium-high speed until the whipped cream forms firm peaks - when you pull the whisk up out of the cream, you will see the “Dairy Queen swirl.” Be careful not to over-whip the cream.
Assemble: Before assembling, set aside a portion of strawberries and whipped cream for decorating if desired. Put an even layer of cake cubes (about 1-2 cubes deep) in the bottom of a trifle dish. Cover with half of the strawberries, followed by half of the whipped cream. Repeat this process once more.
Decorate & Store: Decorate the top of the trifle as desired, cover, and keep refrigerated until needed. Store in the fridge for up to 2-3 days. Extra cake cubes can be frozen for up to 3 months or used to make miniature trifles (see full article for how to do this).
*If using store-bought cake mix, skip Yellow Cake steps 1-6 and follow instructions on box instead.