Preheat your oven to 325°F (165°C). Prepare an 8-inch square baking pan by coating it with nonstick spray, lining it with parchment paper with an overhang on two sides, and lightly spraying the parchment.
Melt the butter in a large microwave-safe bowl. In a separate medium bowl, whisk together the sugar, cocoa powder, flour, baking powder, and salt.
Add the dry ingredients to the melted butter. Stir in the eggs, egg yolk, vanilla, and hot water until just combined. Vigorously stir the batter for about 30 seconds until it becomes smooth and glossy. If using, fold in the mini chocolate chips.
Pour the batter into the prepared pan and spread it into an even layer. Bake for 42 to 45 minutes. The brownies are done when a toothpick inserted into the center comes out with a few moist crumbs attached.
Let the brownies cool completely in the pan on a wire rack, which takes about 1 hour. Then, refrigerate the pan for at least 2 hours to firm up the brownies for cutting.
Use the parchment paper overhangs to lift the chilled brownie slab out of the pan. Place it on a cutting board and cut into 16 equal squares.
Place the white chocolate chips and vegetable oil in a small, microwave-safe bowl. Microwave in 20-second intervals, stirring between each, until the chocolate is melted and smooth.
Dip the bottom of each brownie square into the melted white chocolate, allowing any excess to drip off. Turn the brownie right-side up and place it on a parchment-lined baking sheet.
Immediately sprinkle the top with crushed candy canes before the white chocolate sets. Repeat with the remaining brownies. Allow the topping to set completely at room temperature, about 30 minutes, before serving.