Preheat oven to 350F and grease three 4"x 3" cake rounds*, line with parchment.
In a small bowl whisk flour, baking powder, and salt, set aside.
Cream butter and sugar on med-high until pale and fluffy, approx. 3mins. Add egg and mix until combined. Add vanilla.
Alternate adding flour mixture and milk, beginning and ending with flour (3 additions of flour and 2 of milk), fully incorporating after each addition.
Divide batter evenly amongst pans and bake until a toothpick inserted into the center comes out clean (approx 25-30mins).**
Cool in pans for 10mins, invert and cool completely on wire rack.
Cream butter medium high until pale and fluffy. Reduce speed and slowly add in sugar. Beat until smooth and add vanilla and color gel. Beat until combined.
Using a serrated knife or a cake leveler, cut each cake in half horizontally.***
At this point you can either have three 2-layer cakes or make two 3-layer cakes.
Place a cake layer down and add 1/3 cup buttercream on top, spread evenly. Add 1 tsp jam on top of buttercream and spread evenly. Place another cake layer on top and repeat the process, finishing with buttercream on top and fresh berries.
* This is what I used, hoping to have 3 tall cakes, instead, you could use two 4" x 3" rounds but adjust baking time accordingly. ** Batter will be quite thick. *** Remove any domes first, or prevent them by using bake even strips.Adapted from Rose Levy Beranbaum's The Cake Bible.