Preheat oven to 350°F (175°C) and grease two 9-inch round cake pans. Whisk flour, baking powder, and salt in a bowl. In another bowl, beat 3/4 cup butter and sugar until fluffy, then add eggs one at a time and vanilla. Alternately add flour mixture and 1 cup milk, mixing until just combined. Divide between pans and bake 25 to 30 minutes until a toothpick comes out clean. Cool completely.

Beat 1 cup butter until smooth. Gradually add powdered sugar on low, then beat in vanilla and 2 tablespoons milk until fluffy. Add more milk if needed. Tint most of the frosting with gel color, reserving some white for details.

Level the cooled cakes. Slice one cake in half horizontally. Stack: one thin round, frosting, the whole cake, frosting, the last thin round. Trim about 1 1/2 inches from one short end for the front. Round the back corners. Cut a diagonal wedge from the front section for the windshield. Round the fenders with small cuts. Freeze 15 minutes.

Spread a thin layer of colored frosting over the whole cake for a crumb coat. Chill 15 minutes until firm.

Frost the cake with the remaining colored frosting. Press cookies into the sides for wheels, securing with a dab of frosting. Add yellow M&Ms for headlights and red for taillights. Add a fruit roll-up windshield if desired. Serve at room temperature.