Preheat the oven to 400 F. Line cookie sheet with parchment paper or silicone mat.

To make the dough, combine the butter, oil and brown sugar. Mix for a couple minutes until it is smooth and fluffy. Add the egg and vanilla and mix again. Add the remaining ingredients and mix until just combined.

To make the streusel, combine all the ingredients in a bowl and mix with a spoon or your hand. Start with 1/3 cup of flour. The mixture should stick together, but should also crumble when you run it through your fingers. If it isn't crumbling, add 1 tbsp additions of flour until it crumbles.

Using a large (1/4 cup) cookie scoop, scoop a ball of dough onto a lined baking sheet. Using the back of your cookie scoop or your fingers, make an indent on the top of the dough. Sprinkle a couple tablespoons of the streusel on the top of the cookie, into the indent.

Bake the cookies at 400 for 9-11 minutes. Cool on wire racks.

To make the icing, combine the powdered sugar, milk, and vanilla in a small bowl and whisk until it's smooth and runny. Drizzle on cooled cookies.