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Preheat the oven to 375°F with a rack in the center.
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To make the topping: In a large bowl, combine the sugar and lemon zest. Use your fingertips to rub the zest into the sugar until it smells fragrant.
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Add the flour, baking powder, salt, and baking soda, and whisk until combined.
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Pour the heavy cream into a microwave-safe measuring cup or bowl with at least a 2-cup capacity and heat it in the microwave until it’s hot to the touch (115°F to 125°F), about 1 minute. (Alternatively, heat the cream in a small saucepan over medium heat until hot to the touch.) Whisk the sourdough discard into the hot heavy cream until completely combined.
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Pour the heavy cream mixture into the dry ingredients and mix with a flexible spatula until the mixture is well combined, thick, and has small bubbles throughout. Set aside.
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To make the filling: In an ungreased 8" square pan (both metal or ceramic pans work well), combine the blueberries and lemon juice. Sprinkle the sugar over the blueberries and toss until the berries are evenly coated.
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To assemble the cobbler: Use a generously mounded 2-Tablespoon Scoop or two spoons to scoop the topping in 2-tablespoon portions (about 54g each) onto the filling; arranging the scoops in a 3 x 3 pattern makes a neat presentation. Sprinkle the coarse sparkling sugar evenly over the topping.
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To bake: Bake the cobbler for 40 to 45 minutes, until the top is golden and the filling is bubbling.
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Remove the cobbler from the oven and brush with the melted butter. Let the cobbler cool for 10 minutes before serving. This cobbler is best enjoyed freshly baked. Serve with a dollop of whipped cream or a scoop of vanilla ice cream on top, if desired.
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Storage information: Store leftover sourdough discard cobbler in an airtight container in the refrigerator for up to 1 week. Reheat portions in the microwave for 30 seconds or until warmed through.
