For the Cake:

1. Preheat oven to 350°F. Line three quarter-sized baking sheets with parchment paper. Grease evenly with cooking oil spray, and set aside.
2. In a stand mixer fitted with a paddle attachment, beat the butter at medium-high speed until softened and lightly whipped. Add the salt and sugar. Continue to beat at medium-high speed until well combined.
3. With the mixer running on low speed, add one egg at a time. Increase the speed to medium-high and beat until fully incorporated. Repeat with the other two eggs.
4. Sift together the flour and baking powder in a small bowl. Measure out the milk in a separate measuring pitcher. Measure the vanilla and stir it into the milk. On low speed, add the dry ingredients and liquid mixture alternately to the butter mixture, starting and ending with the dry ingredients. Scrape the bottom and sides of the bowl after each addition, beating well until a uniform batter develops.
5. Divide the batter equally between three separate bowls. Add red food coloring to one bowl, and blue food coloring to the second bowl. Leave the third bowl as is.
6. Stir to mix the dye into the batter. Once the desired color is obtained, transfer each bowl of batter to one of the prepared three baking sheets. Thinly and evenly spread the batter across the baking sheet.
7. Bake each pan for 15-20 minutes, or until the cake springs back gently when touched and a toothpick inserted into the center comes out clean.
8. Cool completely in the pans.

For the Buttercream:

1. In the bowl of a stand mixer fitted with a paddle attachment, mix the softened butter on medium speed until it is smooth with no lumps.
2. Sift the confectioners’ sugar in a separate bowl. Add one cup at a time to the butter, beating very well after each addition.
3. Add the salt, vanilla, and heavy cream. Mix on medium-high speed until very light and fluffy, approximately 5 minutes. Add more heavy cream if mixture is too thick.

To Assemble:

1. Using a small cookie cutter the same size as the circumference of the jar or trifle dish, cut out 12 circles from each of the red, white, and blue cakes.
2. Place one color cake round on the bottom of the jar. Pipe or spread a thick, even layer of buttercream on top. Repeat with the other two cake colors, ending with a layer of buttercream.
3. Decorate with sprinkles on top. Serve and enjoy!