Grease a 9x5 loaf pan and set aside.
Preheat your oven to 350 degrees.
Start off by making the crumble topping. In a medium-sized mixing bowl, whisk together the flour, sugar, salt and cinnamon. Mix in the melted butter and vanilla extract. Whisk together until the butter is well incorporated and the texture is crumbs all about the size of peanuts.
If the mixture turns into a solid dough, use your fingers to break it up. Try to avoid really large clumps as they'll bake into crunchy rocks and make it impossible to slice the cake. Set the crumb mixture aside while you work on the batter for the cake.
In a medium sized bowl, cream the butter and sugar together with an electric mixer on high speed until light and fluffy. Add the egg. Beat in the vanilla, ¼ cup of the dulce de leche and sour cream, scraping down the sides of the bowl with a spatula as needed.
Slowly add in the flour, baking soda, salt and baking powder and mix until combined. Pour half the batter into the prepared pan and spread it out as evenly as possible.
Using a spatula, make an indentation along the center of the cake. This will be the pocket for your dulce de leche filling.
Using a spoon, carefully add the remaining dulce de leche into the indentation. Then, add the remaining cake batter.
Top the entire cake with the crumb mixture. Bake for 50-55 minutes or when the cake passes the toothpick test. Allow to cool completely before slicing.