For cinnamon streusel: Add the flour, sugar, and cinnamon to a small bowl, and whisk to combine. Add the butter to a small saucepan, and heat over low until melted. Pour the butter into the flour mixture, and stir with a small spatula until a crumbly dough is formed. Cover and refrigerate until firm, at least 30 minutes.

Preheat the oven to 350°F (180°C) and place 3 paper muffin liners into a muffin pan.

For muffins: Add the butter to a small saucepan, and heat over low until melted. Pour the butter into a small bowl with the sugar, and stir together. Add the sour cream and vegetable oil, and stir to combine. Add the egg white, and stir to combine.

In a separate bowl, whisk together the flour, cinnamon, baking powder, and salt.

Make a well in the dry goods, and pour in the butter mixture. Stir together to form a batter; don’t worry if it’s a little lumpy, that’s fine.

Divide the batter between the muffin liners, and crumble the streusel atop the batter.

Bake in the preheated oven for 20 to 22 minutes or until the muffins are browned at the edges and the tops spring back to a light touch. Remove from the pan, and set aside for 15 minutes before transferring to a wire rack to cool completely.

Once cooled, use a chopstick to poke a hole in the middle of the muffins, wiggling around slightly to make a little space for the filling. Place the dulce de leche into a piping bag fitted with a small piping tip, and pipe about a tablespoon into each muffin. Dust the muffins with a little confectioners’ sugar. Best served on the day they are made.