Preheat oven to 350°F (175°C), lightly grease a 9×13-inch baking dish.
Cake: whisk flour, baking powder, salt; beat butter and sugar 2–3 min until pale; eggs one at a time, vanilla; alternate dry ingredients with milk and sour cream, mix just until combined; spread in dish, bake 30–35 min until it springs back and a toothpick comes out clean or with a few moist crumbs; cool 15 min.
Filling while cake bakes: strawberries, sugar, lemon juice over medium heat 5 min until soft and juicy; stir in cornstarch slurry, cook 1–2 min until slightly thickened; lightly mash, keep pieces; reserve a few tablespoons.
Poke holes 1 inch apart with a wooden spoon handle while cake is still slightly warm; spoon warm filling over, encouraging it into the holes; cool completely, refrigerate at least 1 hour.
Crumbs: crushed wafers + melted butter on a small baking sheet, bake at 350°F (175°C) 5–7 min until lightly golden; cool completely.
Whipped cream: beat cold cream, powdered sugar, vanilla to medium-stiff peaks, do not overbeat.
Assemble: spread whipped cream over chilled cake, arrange sliced strawberries, spoon reserved sauce between them, finish generously with crumbs.
Chill assembled cake at least 1 additional hour before slicing.