Preheat the oven to 375°. To make the crumble, combine all of the ingredients in a bowl or food processor and mix until it's nice and crumbly.
In a large bowl beat the sugar and melted butter together. Add in the egg, milk, and vanilla and mix until well combined.
Mix the dry ingredients together, and add slowly to the wet ingredients. Mix until just combined.
Line an 8x8 pan with parchment paper, and scoop the batter into it. The batter will be thick. Spread it out evenling in the pan. Top with the crumble mixture and bake in the oven for 25-30 minutes.
While the cake bakes, make the caramel! Combine the butter, sugar and milk in a small saucepan and cook over medium heat stirring constantly. Once the mixture starts to boil, keep cooking for 1-2 minutes. The mixture will thicken. Turn off the heat and add in the vanilla.
Once the cake has cooled for 10 minutes, remove from the pan and continue to let cool. You can either drizzle the caramel on now, or wait until serving! There is a lot of caramel, so use as much to your taste!