1. Mix dry ingredients thoroughly, cut cold butter in with a pastry blender; or alternately, pulse in food processor.
2. Stir ONE Tbsp white vinegar into one cup of milk to sour it. Rest it a couple minutes to let it clot up. Add lightly beaten egg to the milk, and add vanilla (or extract of choice); mix together.
3. Add liquids to the dry ingredients, stir just until blended (don't beat).
4. Grease 9 inch pan, round or square, turn batter out and spread if needed.
5. Sprinkle streusel topping over and bake at 350 until toothpick test passes muster, 30-45 minutes.
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