BLACKBERRY, LEMON, AND THYME MUFFINS

Steps:

• For crumble:
• Whisk first 5 ingredients in a medium bowl. Add butter. Using your fingertips, rub in butter until pea-size lumps form. Add egg yolk; stir to evenly distribute and form moist clumps. (Crumble should resemble a mixture of pebbles and sand.) Chill for at least 1 hour. DO AHEAD: Can be made 3 days ahead. Cover and keep chilled.
• For muffins:
• Preheat oven to 325°F. If making standard-size muffins, line 16 1/3-cup molds with paper liners. Whisk 1 cup all-purpose flour and next 4 ingredients in a medium bowl. Using an electric mixer, beat butter until pale and creamy, about 2 minutes. Add sugar and continue to beat until well incorporated, 2-3 minutes longer. Whisk eggs and vanilla in a small bowl to blend; gradually beat into butter mixture. Continue beating until light and fluffy, 3-4 minutes. Combine buttermilk and lemon zest in a small bowl; gradually beat into butter mixture. Add dry ingredients; beat just to blend (do not overmix).
• Toss blackberries and thyme with 2 tablespoons flour in another small bowl; fold into batter, gently crushing berries slightly to release some juices. Spoon about 2/3 cup batter into large paper muffin molds, or divide between prepared muffin pans. Top each large muffin with 2 tablespoons crumble or each small muffin with 1 rounded tablespoon crumble.
• Bake until tops are golden brown and a tester comes out clean when inserted into center, about 50 minutes for large muffins and 40 minutes for standard-size muffins. Let cool in pan at least 20 minutes, then transfer muffins to a wire rack to cool. Serve warm or at room temperature. DO AHEAD: Can be made 1 day ahead. Store cooled muffins airtight at room temperature.