In a medium bowl, sift together the flour, baking powder, baking soda, and fine sea salt. Set this dry mixture aside.
Using a stand mixer or hand mixer, beat the softened butter on medium speed until smooth, about 1 minute. Add in the vegetable oil and granulated sugar, continuing to beat for another 1–2 minutes until light and fluffy.
Scrape down the sides of the bowl as needed. Incorporate the eggs one at a time, ensuring each egg is fully blended before adding the next. Stir in the vanilla extract followed by the room-temperature milk, mixing just until combined.
Gently fold in the dry ingredients with a rubber spatula, being careful not to overmix to preserve the batter’s lightness.
Evenly divide the batter over the prepared graham cracker crusts, filling each liner approximately two-thirds full. Bake in the preheated oven for 18 minutes, but begin checking for doneness at the 15-minute mark. Lightly press the tops of the cupcakes; if they spring back, they are ready. Allow the cupcakes to cool completely before frosting.