In the bowl of a stand mixer, combine flour, sugar, baking powder, and salt. Add the butter and mix on low until the mixture resembles wet sand, about 2 minutes.

Whisk eggs, milk, and vanilla in a measuring cup. With the mixer on low, slowly pour in the liquid, then increase to medium and beat until smooth, about 1 minute. Gently fold in the 1 cup of jimmies with a spatula.

Preheat oven to 350°F (175°C). Grease and line two 8-inch round pans. Divide batter evenly and bake 25-28 minutes, until a toothpick comes out clean. Cool in pans 10 minutes, then turn out onto a rack.

Beat the 1 1/2 cups butter until creamy, about 3 minutes. Gradually add confectioners’ sugar, salt, vanilla, and 2 tbsp cream. Beat on low until combined, then on high until fluffy, 3-4 minutes, adding more cream if needed. Fold in 1/3 cup jimmies.

Level the cooled cakes if needed. Place one layer on a plate, spread 1 cup frosting, top with the second layer. Crumb coat the cake and chill 20 minutes. Finish frosting, then press extra jimmies onto the sides.

Let the cake sit at room temperature for 30 minutes if chilled, then slice and serve.