Snippet to add your own button inside Unreal's blueprint editor toolbar

Unreal Engine - Extend blueprint editor toolbar

void FYourModuleModule::StartupModule()
{
	UToolMenu* ToolBarMenu = UToolMenus::Get()->ExtendMenu("AssetEditor.BlueprintEditor.ToolBar");
	FToolMenuSection& Section = ToolBarMenu->AddSection("MySection", INVTEXT("My Section Name"));
	Section.AddEntry(FToolMenuEntry::InitToolBarButton(
		FName("MyAction"),
		FUIAction(FExecuteAction::CreateLambda(
			[]()
			{
				UE_LOG(LogTemp, Warning, TEXT("MyAction was executed"));
			}
		)),
		INVTEXT("My Action Name"),
		INVTEXT("My Action Tooltip"),
		FSlateIcon(FName("OutputLogStyle"), "Log.TabIcon")
	));
}