01 - Preheat oven to 375°F. Line an 8x8-inch baking pan with parchment paper, allowing excess to hang over two sides for easy removal.
02 - In a large mixing bowl, whisk together all-purpose flour, baking powder, baking soda, and fine sea salt.
03 - Cut cold unsalted butter into the flour mixture using a pastry cutter or fork until coarse crumbs form.
04 - Pour in cold buttermilk and the beaten egg. Stir just until the dough comes together; avoid overmixing.
05 - Turn dough onto a floured surface. Lightly pat into a rectangle approximately 1/2-inch thick. Fold dough in thirds, rotate 90 degrees, and repeat folding once more.
06 - Press or roll dough into a rectangle matching the dimensions of the prepared baking pan. Carefully transfer dough to pan.
07 - Brush dough with melted butter. Combine granulated sugar and ground cinnamon, then sprinkle evenly over the surface.
08 - Press crushed mini salted pretzels lightly into the cinnamon sugar layer.
09 - In a small bowl, whisk together powdered sugar, pure maple syrup, bourbon, vanilla extract, and a pinch of salt until smooth.
10 - Drizzle half of the maple bourbon glaze evenly over the pretzel layer.
11 - Bake in preheated oven for 28 to 32 minutes, or until biscuit layer is golden and fully puffed. Allow to cool in pan for 10 minutes.
12 - Lift biscuits out using parchment overhang. Drizzle with remaining glaze. Let cool completely before slicing into bars.