Weird air pockets/bubbles on bottom of cake I've been baking this recipe for over a year now, flavour and texture are wonderful, the guests love it but half of the time I get these air pockets on the bottom of the cake.
The air pockets appear at the center of the bottom and also at the sides. Sometimes they're barely there, sometimes they're big enough that I'm unsure if i'll serve it to guests (for exemple when they're 3cm height).
  
Recipe is as follows:

150g cashew nuts (roasted, unsalted) chopped in a food processor
30g white flour
125g sugar
pinch of salt
4 egg whites, lightly beaten with a fork
150g butter, heated untill brown
1/2 teaspoon vanilla extract

Everything at room temperature.
Preparation is staightfoward: mix dry ingredients, add egg whites, add butter and vanilla. Cooks at 160~180C for 45~50m untill golden brown. Lower temperatures do not make the bubbles disappear.
What causes these air pockets? Has anyone faced a similar issue before?
That happened to me as well. Luckily it doesn't change the taste, but it is annoying! It comes from air pockets which formed because you whisked/mixed your cake batter too intensely, and/or for too long, which creates air bubbles. So first, it is best to mix your ingredients delicately, try to avoid incorporating too much air (easier said than done), and stop whisking once your batter reached the desired homogeneous consistency (overwhisking is a common mistake). Then, when you pour your cake batter in the pan, it's better to avoid pouring it all at once in the center, it will also create air bubbles. It's better to pour it delicately from one corner and let it spread by itself in the dish. Finally, before putting in the oven, tap (delicately) the bottom of your pan on the counter to free the remaining air bubbles and let them reach the surface before cooking.
I hope that will help, let me know!