Preheat the oven to 350°F (175°C).

Grease the inside of each mason jar with cooking spray or butter and set aside.

Sift together the flour, baking powder, and salt in a medium mixing bowl. Set aside.

Cream together the softened butter and granulated sugar in a large mixing bowl, until light and fluffy using a hand mixer or stand mixer.

Add the eggs one at a time, beating well after each addition. Then, mix in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk, starting and ending with the dry ingredients. Mix until just combined, being careful not to overmix.

Divide the batter evenly among the prepared mason jars, filling each jar about halfway full.

Place the filled mason jars on a baking sheet and transfer them to the preheated oven.

Bake for 25-30 minutes, or until a toothpick inserted into the center of a cake comes out clean.

Remove the jars from the oven and allow them to cool completely on a wire rack.

Once cooled, spread a layer of frosting on top of each cake in the jar. Add your favorite toppings to decorate.

Screw the lids onto the jars and store them in the refrigerator until ready to serve.