Preheat oven to 350°F. Grease the bundt pan and sprinkle with flour.
In a large bowl, beat butter and sugar until light and fluffy.
Add the eggs, one at a time, and mix to combine.
Add the canola oil, mix until combined. Then add the milk and vanilla.
Add the flour, baking powder, and salt and mix just until combined.
Divide the cake batter in half. Mix half of the dough with the cocoa powder. Once that's incorporated, add the milk and mix it in as well.
Using an ice cream scoop (or a 1.4 cup measuring cup), pour two scoops of vanilla mixture into the pan. Let it spread for a few seconds. Then pour 2 scoops of the chocolate batter on top of the vanilla, right in the middle. Let that batter spread slightly. Continue to alternate between vanilla and chocolate layers until the bundt pan is filled.
Bake the bundt cake for 50-60 minutes. Cover with foil in the final 10 minutes, if necessary. A toothpick inserted into the center of the cake should come out clean.
Allow the cake to cool in the pan for 10 minutes before overturning onto a wire rack to cool completely.
To make the glaze, put the chocolate and butter in a microwave-safe bowl and microwave for about 40-60 seconds, or until the chocolate and butter are melted and smooth. Stir the powdered sugar into the melted butter and chocolate. Beat the mixture, adding small amounts of hot water until thinned as desired.
Pour the glaze over the bundt cake.