Step 1: Prepare the Oven and Pans
Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, or grease and flour two 8-inch round cake pans. The parchment makes removing the cake much easier for cutting into jar-sized pieces.

Step 2: Mix the Dry Ingredients
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Whisk thoroughly for about 30 seconds to ensure even distribution of the leavening agents. This prevents pockets of baking powder or soda that could create an uneven rise.

Step 3: Cream Butter and Sugar
In a large bowl using an electric mixer, beat the softened butter and granulated sugar on medium-high speed for 3-4 minutes until light, fluffy, and pale in color. This creaming process incorporates air for a lighter cake texture. Scrape down the sides of the bowl as needed.

Step 4: Add Eggs and Vanilla
Add the eggs one at a time, beating well after each addition (about 30 seconds per egg). Mix in the vanilla extract. The mixture should look smooth and slightly thickened.

Step 5: Alternate Wet and Dry Ingredients
With the mixer on low speed, add the flour mixture in three additions, alternating with the buttermilk in two additions (begin and end with flour). After each addition, mix just until barely combined—you should still see streaks of flour. Add the vegetable oil and mix on low for 10-15 seconds until just incorporated. Don’t overmix, as this develops gluten and creates a tough cake.

Step 6: Bake the Cake
Pour the batter into your prepared pan(s), smoothing the top with a spatula. Tap the pan gently on the counter a few times to release air bubbles. Bake for 28-32 minutes (9×13 pan) or 25-28 minutes (round pans) until a toothpick inserted in the center comes out clean or with just a few moist crumbs. The cake should spring back when lightly pressed and pull away slightly from the pan edges.

Step 7: Cool Completely
Let the cake cool in the pan for 10 minutes, then turn out onto a cooling rack. Allow it to cool completely before cutting—this takes about 1-1½ hours. Cutting warm cake will result in crumbly pieces that don’t layer well.

Step 8: Make the Cream Cheese Frosting
While the cake cools, beat the softened cream cheese and butter together on medium-high speed until completely smooth and fluffy, about 2-3 minutes. Add the powdered sugar one cup at a time, mixing on low speed after each addition. Add vanilla extract and salt. Finally, add heavy cream one tablespoon at a time until you reach a smooth, pipeable consistency. Beat on high for 1-2 minutes until light and fluffy.

Step 9: Cut the Cake
Once the cake is completely cool, cut it into small cubes or circles that fit nicely in your jars. For 8 oz jars, cut 1-inch cubes or use a 2-inch round cutter. You’ll need enough pieces to create 2-3 layers in each jar.

Step 10: Layer the Jars
Place a layer of cake pieces at the bottom of each jar, pressing down very gently. Pipe or spoon a layer of frosting over the cake (about 2-3 tablespoons). Add another layer of cake, followed by more frosting. Repeat for 2-3 layers, depending on your jar size. The top layer should be frosting, smoothed nicely with an offset spatula.

Step 11: Add Toppings and Serve
Top each jar with your chosen toppings—fresh berries, chocolate chips, cookie crumbs, or a drizzle of sauce. Garnish with a sprig of mint if desired. Seal the jars with lids if making ahead, or serve immediately.