1. The cake. Preheat the oven to 350 degrees. Butter a 17 x 12-inch half sheet pan, then line with parchment paper and butter and flour the paper.
Whisk together the flour, baking soda, baking powder and salt. Place the softened butter and the sugar in a mixing bowl. Using the paddle attachment of an electric mixer, beat on medium speed until light and fluffy. Add the eggs, one at a time, beating each until just combined. Mix in the vanilla. Add the flour mixture alternating with the buttermilk in 3 stages. Mix until each addition is incorporated, taking care not to overbeat. Scrape the cake batter into the prepared sheet pan and smooth the top with a spatula. Place on the middle rack in the oven and bake for 30 to 35 minutes or until golden brown and the center springs back. Let cool in the pan for 20 minutes, then turn out onto a cooling rack. Once completely cool, cut out twelve 4-inch circles using a biscuit cutter. Place the cake circles on a parchment lined rimmed baking sheet. (If you're working ahead, wrap well and freeze.) Using a wooden skewer, poke holes all over the surface of the cake. This will help the cake absorb the tres leches mixture more evenly.
2. Tres Leche. Combine the milk, evaporated milk, and sweetened condensed milk in an 8 x 8 baking dish. Dip each cake circle into the tres leches mixture, gently flipping the cake to ensure that each piece has evenly absorbed the liquid. Place the soaked cake back onto the baking sheet until you're ready to assemble the dessert. You can do this step earlier in the day, just wrap and refrigerate.
3. Strawberries. Scoop the sliced strawberries into a bowl, sprinkle the sugar over the top along with the raspberry liqueur or orange juice and gently stir to combine. Allow the strawberries to marinate for 1 hour, stirring occasionally. Strain the strawberries, reserving the syrup separately.
4. Finishing the shortcakes. When you're ready to assemble, using a metal spatula, transfer one of the cake circles onto a plate. Smear a dollop of whipped cream over the top, pile on the strawberries and drizzle on some of the reserved syrup.