First, brown the butter. Place 4 tablespoons unsalted butter in a small saucepan over medium heat and whisk gently, as it continues to melt. Continue whisking gently as it bubbles. After 2 to 3 minutes, you should see brown bits appear at the bottom of the pan, the surface will foam, and the butter will emit a nutty aroma. Remove from heat immediately and continue whisking for another 30 seconds, before setting aside on a wire rack to cool slightly.
In a medium bowl, whisk together 2 large eggs and 1/4 teaspoon salt until the egg looks uniformly yellow. Add about 1/3 cup of the all-purpose flour and whisk into the eggs until just combined — at this point the batter will seem lumpy, but that's okay. Add about half of the 1 cup plus 2 tablespoons whole milk into the batter and continue whisking until the mixture thins. Keep adding the milk and flour alternately until you've added it all. Continue whisking until the batter is just smooth, before adding the butter. Continue whisking until the butter has fully incorporated into the batter. At this point, you're ready to cook the crepes.
Preheat a medium nonstick skillet pan with a handle on medium heat. Once the pan is hot enough (you can tell by putting your hand over the surface and seeing if it emits heat), pour about 1/4 to 1/3 cup of batter into the pan. As you pour the batter, use the handle to twirl the batter around. The batter will coat the pan in a circle to make the crepe. Once you're satisfied with the crepe you've formed, place the pan back over its heat source. As it cooks, it will change in appearance — you'll see the edges start to curl upwards and take on a lacy texture. Once unsticking the edges, a cooked crepe should slide easily around the pan.
Once one side is ready, slip a spatula under the crepe and flip it over. Continue cooking for another 20 to 30 seconds, before transferring onto your serving plate. The crepes are best when warm and fresh, so go ahead and layer on as much Nutella and Speculoos as you desire. Both fillings will get a little bit melty and delicious. Sprinkle with banana slices. Fold up into a half, and fold that up into another half to get a triangle. Eat immediately!
Repeat until you've run out of crepe batter.