Spray a 9×13-inch baking pan with nonstick spray. Line with parchment paper, leaving overhang on the long sides, then spray the parchment.
In a measuring cup, combine yeast and warm milk. Let sit 5–10 minutes until foamy.
In a large bowl, cream softened butter, sugar, and eggs until light and fluffy.
Mix in orange zest.
Add flour and salt and mix until just combined.
Pour in the yeast mixture and beat on high speed for 1–2 minutes until the dough becomes sticky and elastic, resembling a soft bread dough.
Spread dough evenly into the prepared pan using an offset spatula.
Cover with plastic wrap and let rise at room temperature for 1 hour.
Meanwhile, brown the butter for the topping. Melt ½ cup butter in a saucepan over medium heat, stirring continuously until golden brown and nutty smelling, about 5–7 minutes. Cool slightly.
Preheat oven to 350°F.
In a large bowl, combine softened butter, browned butter, sugar, vanilla, corn syrup, egg, egg yolk, salt, bourbon, and flour. Mix until smooth and thick.
Drop spoonfuls of topping over the risen dough and gently spread with an offset spatula without pressing down too firmly.
Bake for 30–35 minutes until the top is golden brown and the edges are deep golden. The center should still look gooey.
Cool completely in the pan.
Use parchment overhang to lift cake from the pan. Dust with powdered sugar, cut into squares, and serve.